1. NJI
Physics 320: Astronomy and
Astrophysics – Lecture I
Carsten Denker
Physics Department
Center for Solar–Terrestrial Research
2. NJIT Center for Solar–Terrestrial September 3, 2003
Introduction
History of Solar Physics
Prehistoric era
Ancient Greek
Paradigm shift in planetary models
“Modern” Solar Physics
Why bother … ?
Most of the material has been stolen from
the HAO Education Pages by Paul
Charbonneau (NCAR, HAO)
4. September 3, 2003NJIT Center for Solar–
Solar Observations BC
3 May 1375 BC or 5 March 1223 BC: eclipse
record on clay tablet uncovered in the ancient
city of Ugarit, Syria
8th
century BC: Babylonians were keeping a
systematic record of solar eclipses, predictions
based on numerological rules
800 BC: Oldest record of a sunspot observations
are found in the Book of Changes, China
250 BC: Measurement of the distance to the Sun
by Aristarchus of Samos (ca. 310-230 BC).
5. September 3, 2003NJIT Center for Solar–
Ancient Greek
Physical (geocentric) model of
the cosmos by Aristotle (384 –
322 BC)
Mathematical model of
planetary motion by Ptolemy
(100 – 170), terrestrial/celestial
sphere, basic elements: earth,
water air, and
fire/quintessence
The Aristotelian cosmos. The Earth
sits motionless at the center of the
universe, and the outer sphere, the
Primum Mobile, is assumed to
undergo a full revolution in 24 hours.
6. September 3, 2003NJIT Center for Solar–
Early Observations of the Corona
Report of solar eclipse
observations by the Byzantine
historian Leo Diaconus (950 –
994) on December 22nd
, 968 from
Constantinople (now Istanbul,
Turkey).
Possible eclipse record on oracle
bones dating from the Shang
dynasty in China (1766 – 1123
BC)
Chronicle of Novgorod describes a
prominence during the May 1st
,
1185 solar eclipse: "In the evening
there as an eclipse of the sun. It was
getting very gloomy and stars were seen
... The sun became similar in appearance
to the moon and from its horns came out
somewhat like live embers."
Annales Sangallenses:
"...at the fourth hour of the day ...
darkness covered the earth and all the
brightest stars shone forth. And is was
possible to see the disk of the Sun, dull
and unlit, and a dim and feeble glow like
a narrow band shining in a circle
around the edge of the disk".
7. September 3, 2003NJIT Center for Solar–
Sunspot Observations
Official records of the
Chinese imperial courts
starting in 165 BC
Theophrastus (374 –287 BC)
including details of umbra
and penumbra
Aristotelian views
concerning the
incorruptibility of the
heavens meant that sunspots
were "physically impossible",
sightings were ignored or
ascribed to transit of Mercury
or Venus across the solar disk
From the Chronicles of John of Worcester:
one of the first surviving sunspot drawing
from a sighting on December 8th
, 1128.
"... from morning to evening, appeared
something like two black circles within
the disk of the Sun, the one in the upper
part being bigger, the other in the lower
part smaller. As shown on the
drawing."
8. September 3, 2003NJIT Center for Solar–
Nicholas Copernicus
(1473–1543)
De Revolutionibus
Orbium Coelestum in 1543
Heliocentric planetary model:
The Sun is at the center of all planetary
motions, except for the Moon which
orbits Earth. Under this arrangement
the orbital speed of planets decreases
steadily outwards, and the outer sphere
of fixed stars is truly motionless. In
Copernicus' original model the Earth
has three motions: a daily 24-hr axial
rotation, a yearly orbital motion about
the Sun, and a third motion, somewhat
related to precession which Copernicus
thought necessary to properly
reproduce ancient observations.
9. September 3, 2003NJIT Center for Solar–
Orbital Paths of Planets
Collection of 20 years
of accurate planetary
positions by Tycho
Brahe (1546 – 1601)
Johannes Kepler (1571
– 1630)
1609: Astronomia
Nova
1619: Harmonice
Mundi
1627: Rudolphine
Tables
10. September 3, 2003NJIT Center for Solar–
Galileo Galilei (1564 – 1642)
First telescopic
observations
of the Sun!
11. September 3, 2003NJIT Center for Solar–
Sun as a Star – Maunder Minimum
René Descartes (1596 –
1650) describes the Sun as
a star in his 1644 book
Principia Philosophiae
Maunder minimum 1645
–1715: sunspots vanish
even though a systematic
solar observing program
was underway under the
direction of Jean
Dominique Cassini (1625
– 1712) at the newly
founded Observatoire de
Paris
12. September 3, 2003NJIT Center for Solar–
Isaac Newton (1642 – 1727)
1686: Principia
Mathematica, universal
law of gravitation
Stable planetary orbits
result from a balance
between centripetal and
gravitational acceleration
Sun–to–Earth mass ratio
(MEarth
/MSun
= 28700 instead
of 332945), wrong value
for solar parallax, better
estimate in later edition
of the Principia (within
factor of two)
13. September 3, 2003NJIT Center for Solar–
Infrared Radiation
In 1800, William Herschel (1738 –1822) extended Newton's
experiment of separating chromatic light components via refraction
through a glass prism by demonstrating that invisible "rays" existed
beyond the red end of the solar spectrum.
14. September 3, 2003NJIT Center for Solar–
Spectroscopy
The English chemist and
physicist William Hyde
Wollaston (1766 – 1828)
noticed dark lines in the
spectrum of the Sun while
investigating the refractive
properties of various
transparent substances
Joseph von Fraunhofer
(1787-1826) independently
rediscovered the “dark
lines” in the solar
spectrum
15. September 3, 2003NJIT Center for Solar–
Chemical Composition of the Sun
Reproduction of part of the map of the solar spectrum published in 1863 by
Kirchhoff, showing the identification of a large number of spectral lines with
various chemical elements. Note numerous clear matches for Iron (Fe).
17. September 3, 2003NJIT Center for Solar–
The First Solar Photograph 1845
The first successful
daguerrotype of the
Sun, reproduced
below, was made on
April 2nd
, 1845 by
the French
physicists Louis
Fizeau (1819-1896)
and Léon Foucault
(1819-1868). The
exposure was 1/60
of a second. This
image shows the
umbra/penumbra
structure of
sunspots, as well as
limb darkening.
18. September 3, 2003NJIT Center for Solar–
Sunspot Numbers
Statistics of sunspot number
by Swiss astronomer Rudolf
Wolf (1816-1893)
Relative sunspot number:
r = k (f + 10 g),
where g is the number of
sunspots groups visible on the
solar disk, f is the number of
individual sunspots (including
those distinguishable within
groups), and k is a correction
factor that varies from one
observer to the next (with k = 1
for Wolf's own observations)
Sunspot drawings by Johann
Hieronymus Schroeter (1745 – 1816), an
active solar observer between 1785 and
1795. Schroeter's sunspot drawings
were a primary source for Wolf's
reconstruction of activity cycle number 4
(1785 –1798)
19. September 3, 2003NJIT Center for Solar–
Differential Rotation
Richard C. Carrington
(1826 – 1875)
Spörer's Law of sunspot migration. The thick lines shows the latitude] at which
most sunspots are found (vertical axis, equator is at zero), as a function of time
(horizontal axis). The dashed line is the Wolf sunspot number, showing the rise
and fall of the solar cycle.
Gustav Spörer (1822 –
1895)
20. September 3, 2003NJIT Center for Solar–
First Observation
of a Solar Flare 1859
On September 1st
, 1859, the astronomer R. C. Carrington was engaged in
his daily monitoring of sunspots, when he noticed two rapidly brightening
patches of light near the middle of a sunspot group he was studying.
21. September 3, 2003NJIT Center for Solar–
First Observations of a Coronal
Mass Ejection 1860
22. September 3, 2003NJIT Center for Solar–
The Magnetic Nature
of Sunspots 1908
The magnetically–induced Zeeman splitting
in the spectrum of a sunspot. Reproduced from
the 1919 paper by G.E. Hale, F. Ellerman, S.B.
Nicholson, and A.H. Joy (in The
Astrophysical Journal, vol. 49, pp. 153–178).
George Ellery Hale
(1868–1938)
23. September 3, 2003NJIT Center for Solar–
The Celestial Sphere
Greek Tradition
Copernican
Revolution
Positions on the
Celestial Sphere
Physics and
Astronomy
24. September 3, 2003NJIT Center for Solar–
Positions on the Celestial Sphere
[ ]
[ ]
sin tan
cos
m n N
n N
α α δ
δ α
∆ = +
∆ =
Reference Epoch 1950:
m = 3.07327s
yr –1
n = 20.0426’’ yr –1
Earth precession period is 25,770 years.
25. September 3, 2003NJIT Center for Solar–
Spherical Geometry
sin sin sin
sin sin sin
a b c
A B C
= =
Law of sines:
Law of cosines for sides:
cos cos cos sin sin cosb c b c Aα = +
Law of cosines for angles:
cos cos cos sin sin cosA B C B C a= − +
26. September 3, 2003NJIT Center for Solar–
Proper Motion
d v t
vd
t
r r
vd
dt r
θ
θ
θ
θ
θ
µ
∆ = ∆
∆
∆ = = ∆
≡ =
vθ: transverse or tangential
velocity
vr: radial velocity
27. September 3, 2003NJIT Center for Solar–
Synodic and Sidereal Period
1/ 1/
1/
1/ 1/
Earth
Earth
P P
S
P P
−
=
−
(inferior)
(superior)
Planet Sidereal Orbital
Period [yr]
Mercury 0.2408
Venus 0.6152
Earth 1.0000
Mars 1.8809
Jupiter 11.862
Saturn 29.458
Uranus 84.014
Neptune 164.79
Pluto 248.54
31. September 3, 2003NJIT Center for Solar–
Physics and Astronomy
Astronomy = natural extension of human
curiosity in its purest form
Paradigm shifts
Physical causes for observable phenomena
Astronomy + Physics = Astrophysics
Observations analyze photons and
particles
Tools: telescopes, post–focus
instrumentation, and computers
32. September 3, 2003NJIT Center for Solar–
Homework
Homework is due Wednesday September
10th
, 2003 at the end of the lecture!
Homework assignment: Problems 1.5, 1.6,
and 1.7
Late homework receives only half the
credit!
The homework is group homework (2–3
students)!
Homework should be handed in as a text
document!
Editor's Notes
There is nothing quite like Stonehenge anywhere in the world and for 5000 years it has drawn visitors to it. Standing like ‘soldiers of time,' we shall never know what drew people here over the centuries or why hundreds of people struggled over thousands of years to build this monument, but visitors from all over the world come to marvel at this amazing feat of engineering. To this day Stonehenge remains a mystery. Stonehenge's mysterious alignment with the sun Stonehenge's orientation on the rising and setting sun has always been one of its most remarkable features. Whether this was simply because the builders came from a sun-worshipping culture, or because - as some scholars have believed - the circle and its banks were part of a huge astronomical calendar, remains a mystery. History of Stonehenge Before Stonehenge was built thousands of years ago, the whole of Salisbury Plain was a forest of towering pines and hazel woodland. Over centuries the landscape changed to open chalk downland. What you see today is about half of the original monument, some of the stones have fallen down, others have been carried away to be used for building or to repair farm tracks and over centuries visitors have added their damage too. It was quite normal to hire a hammer from the blacksmith in Amesbury and come to Stonehenge to chip bits off. As you can image this practice is no longer permitted! Origins of Stonehenge and Aubrey Holes The first 'Stonehenge' consisted of a circular bank and ditch with a ring of 56 wooden posts, now known as Aubrey Holes. Later monuments all used, and reused, the great stones we see today, which were brought from some distance away. The final phase comprised an outer circle of huge standing stones - super-hard sarsens, from the Marlborough Downs - topped by lintels making a continuous ring. Inside this stood a horseshoe of still larger stones, five pairs of uprights with a lintel across each pair, known as trilithons. Stones were connected using mortice and tenon and tongue and groove joints, possibly copying previous wood construction techniques. Smaller bluestones, from the Preseli Mountains in South Wales, were arranged in a circle and a horseshoe within the great sarsen stone circle and horseshoe. In an earlier phase these bluestones had been erected in a different arrangement. There has always been intense debate over quite what purpose Stonehenge served. Certainly it was the focal point in a landscape filled with prehistoric ceremonial structures. It also represented an enormous investment of labour and time. A huge effort and organisation was needed to carry the stones tens, sometimes hundreds of miles by land and water, and then to shape and raise them. Only a sophisticated society could have organised so large a workforce and possessed the design and construction skills necessary to produce Stonehenge and its surrounding monuments. What cannot be denied is the ingenuity of the builders of Stonehenge. With only very basic tools at their disposal, they shaped the stones and formed the mortices and tenons that linked uprights to lintels. Using antlers and bones, they dug the pits to hold the stones and made the banks and ditches that enclosed them.There are direct links with the people who built Stonehenge in their artefacts, tools, pottery and even the contents of their graves. Some of these are displayed in the museums at Salisbury and Devizes.
Total eclipses of the Sun are arguably the most impressive astronomical phenomenon that can be observed more or less regularly with the naked eye. They occur when the Moon reaches a point in its orbit around the Earth that lies on the line joining the Earth and Sun. By a remarkable coincidence, the Moon's angular diameter, as seen from the Earth, is almost identical to that of the Sun. The Sun's disk is then completely eclipsed, and daytime darkness falls upon the Earth for a few minutes (This physical explanation of the phenomenon was only put forth much later, in the first century BC). Like comets, solar eclipses were taken to be astrological omens of great significance. It is therefore not surprising that such a spectacular event is often mentioned in surviving written records and chronicles of ancient civilizations. The oldest eclipse record is found on a clay tablet uncovered in the ancient city of Ugarit, (in what is now Syria), with two plausible dates usually cited: 3 May 1375 BC or 5 March 1223 BC, the latter being favored by most recents authors on the topic. It is certainly clear that by the eight century BC, the Babylonians were keeping a systematic record of solar eclipses, and may even have been able to predict them fairly accurately based on numerological rules. Sunspots are concentrations of strong magnetic fields piercing the solar photosphere. Visually, they look like dark blemishes on the solar disk. Most sunspots are too small to be readily visible by naked eye observations, but some reach a size sufficient to be visible without a telescope, under suitable viewing conditions (for example, when the sun is partially obscured by fog or thick mist, or clouds). Because of their possible astrological significances, reports of naked-eye sunspot observations are indeed to be found in many ancient chronicles and court chronologies. The two oldest record of a sunspot observation are found in the Book of Changes , probably the oldest extant Chinese book, compiled in China around or before 800 BC. The text reads "A dou is seen in the Sun" , and A mei is seen in the Sun" . From the context, the words (i.e., chinese characters) " dou " and " mei " are taken to mean darkening or obscuration. Astronomers at the court of the Chinese and Korean emperors made regular notes of sunspots, most less elliptical than the one cited above. It seems, however, that observations were not carried out systematically for their own sake, but instead took place whenever astrological prognostication was demanded by the emperor. The surviving sunspots records, though patchy and incomplete, covers nearly 2000 years and represents by far the most extensive pre-telescopic sunspot record.
One of the major intellectual achievement of ancient Greece is the physical model of the cosmos developed by Aristotle (384-322). An essential feature is the place occupied by the Earth at the center of the Universe, with the Sun, planets and sphere of fixed stars revolving about that center, the Sun occupying the fourth sphere. In this geocentric model the Earth is absolutely fixed, with the motions of precession and daily rotation ascribed to the two outermost spheres of the model. This basic planetary arrangement formed the basis of mathematical model of planetary motion developed four centuries later by Claudius Ptolemy (ca. 100-170). In Aristotle's scheme there exist fundamental physical differences between the terrestrial and celestial realms, as demarcated by the Moon's sphere. Everything under the Moon is made of the four elements earth, water, air and fire, themselves arranged concentrically about the center of the universe. Pure circular motion prevail throughout the heavens, which are are made of an incorruptible fifth element (or "quintessence"). Aristotle was born in 384BC in Stagirus, Macedonia, where his father was court physician to the King. He was sent to Athens in 367 BC to study at Plato's Academy. In 342 BC he was invited back to Macedonia to tutor the crown prince Alexander (who later left his mark on history as Alexander the Great). Around 335 BC he returned to Athens and founded his Lyceum. He died in Chalcis, north of Athens, in 322 BC. The breadth (and volume!) of Aristotle's writings is staggering by any standard. He wrote on philosophy, logics, politics, biology, physics and cosmology. In his On the Heavens ( De caelo ), Aristotle adopted with some modifications the geocentric planetary model of Eudoxus (ca. 400-347 BC) and Callipus (ca. 370-300BC), but ascribed physical reality to the planetary spheres. His physics relies on an essential distinction between the sublunar realm, made of the four elements earth, water, wind and fire, and the celestial realm, made of ether (or "quintessence") and deemed incorruptible. Aristotle offered the world an internally consistent physics and cosmology of hitherto uncomparable breadth and explanatory power, which was to endure for more than 1200 years. In conjunction with Ptolemy 's mathematical model of planetary motions, it was to form the cornerstone of the christian medieval view of the cosmos. Little is known about Ptolemy's life and education, other that he lived and worked in Alexandria, one of the primary centers of Greek culture in late antiquity, and that he has no genealogical relationship whatsoever to the Pharaoh dynasty bearing the same name (as believed by many in medieval times). He wrote on many topics, including geography, astrology, musical theory, optics, physics, and of course astronomy. He died most probably between 141 and 151 AD. His landmark astronomical work is his Mathematical compilation , (or Syntaxis , as Prolemy himself referred to it) better known under the name Amalgest , given to it by its later Arabic translators and commentators. By making extensive use of the geometrical constructions known as epicycles and equants , Ptolemy constructed a mathematical model of planetary motion that did far better at predicting planetary positions than anything else produced in antiquity. It is the fusion of Ptolemy's model with the cosmology and physics of Aristotle that was to be adopted in the late antiquity and medieval Western world, and literally define mankind's view of the universe for over a millennium. Ptolemy was also an accomplished geographer. His maps of Asia and Africa are said to have inspired Christopher Columbus, many centuries later, in his westward expedition to India across the Atlantic.
This drawing, from the Chronicles of John of Worcester (one of the many monks who contributed to the Worcester Chronicles), represents to the best of our knowledge the first surviving sunspot drawing, from a sighting on Saturday, 8 December 1128. The accompanying text translates to something like: "...from morning to evening, appeared something like two black circles within the disk of the Sun, the one in the upper part being bigger, the other in the lower part smaller. As shown on the drawing." The facts that the Worcester monks could apparently distinguish the umbrae and penumbrae of the sunspots they observed suggests that the spots must have been truly exceptionally large. Large sunspots can be visible to the naked-eye under suitable viewing conditions, for example when the sun is partially obscured by clouds or mist, particularly at sunrise or sunset. Numerous such sighting exist in the historical records, starting with Theophrastus (374-287 B.C.) in the fourth century B.C. However, by far the most extensive pre-telescopic records are found in the far east, especially in the official records of the Chinese imperial courts, starting in 165 B.C. In the West, the dominating views of Aristotle concerning the incorruptibility of the heavens meant that sunspots were "physically impossible", so that sightings were ignored or ascribed to transit of Mercury or Venus across the solar disk.
The cosmos of the late Christian medieval era was a fusion of ideas combining the physics of Aristotle and the planetary astronomy of Ptolemy . This is the world view that was destroyed in the sixteenth and seventeenth centuries. The first blow was dealt by Nicholas Copernicus (1473-1543), who published his landmark book De Revolutionibus Orbium Coelestium in 1543. There Copernicus presented a new planetary model with the Sun placed in center, and letting all planets (including the Earth) orbit around the Sun. Copernicus also gave the Earth two additional motions: a daily axial rotation, and a precession of that spin axis. In doing so, Copernicus eliminated the need for the two outermost spheres of the ptolemaic model and produced a system where the speeds of revolution decrease gradually outward all the way to the fixed stars. The Copernican planetary model. The Sun is at the center of all planetary motions, except for the Moon which orbits Earth. Under this arrangement the orbital speed of planets decreases steadily outwards, and the outer sphere of fixed stars is truly motionless. In Copernicus' original model the Earth has three motions: a daily 24-hr axial rotation, a yearly orbital motion about the Sun, and a third motion, somewhat related to precession which Copernicus thought necessary to properly reproduce ancient observations. Copernicus ostensibly introduced his heliocentric model in order to do away with equants and various motions previously attributed to the sphere of fixed stars, but it appears clear that he believed in the physical reality of his heliocentric hypothesis. Because Copernicus' model could be construed as yet another mathematical device useful in astronomy but without physical reality, his model could at first be used by astronomers without attracting the ire of philosophers and theologians committed to the centrality and fixity of the Earth. This situation was to change in the next century. Nicolaus Copernicus [Nikklas Koppernigk] (1473-1543). Born on 19 February 1473 in Torun, Poland. He first studied at Cracow from 1492 to 1494, then in 1496 was sent to Italy to study Canon Law at the University of Bologna. In 1501 he began medical studies at the University of Padua, and finally took his Law degree at the small University of Ferrara in 1503. In 1497, while still in Italy, he was made Canon of the Frombork [Frauenberg] cathedral by his maternal uncle and protector Lucas Watzenrode, bishop of Varmia. This provided Copernicus with a secure and relatively renumerative position which he held to the end of his life, allowing him the freedom to pursue his interest in astronomy. Copernicus' landmark work On the Revolutions ( De revolutionibus orbium coelestium ) was dedicated to Pope Paul III and published in 1543 in Nurenberg, as Copernicus lay on his deathbed. However, his heliocentric hypothesis had been circulating for over 30 years, starting with his Commentariolus , written between 1512 and 1515 and circulated in manuscript form. Georg Joachim Rheticus (1514-1574), who joined Copernicus in Frombork in 1539 as his first and only disciple, published in 1540 his Narratio Prima , a first account of the Copernican planetary model. Copernicus died on May 24 1543 in Frombork.
An early convert to the Copernican system was Johannes Kepler (1571-1630). After ten years of laborious work using the accurate observations of planetary positions accumulated over 20 years by the astronomer Tycho Brahe (1546-1601), Kepler came to realize that the orbital paths of planets has the form of ellipses with the Sun at one focus, and that the radius vector joining a given planet to the Sun sweeps equal areas in equal time (today known as Kepler's first and second laws). In 1609 Kepler published his landmark Astronomia Nova , and in 1619 his Harmonice mundi , where what is now known as Kepler's third law (orbital period squared proportional to mean distance cubed) is first laid out. Using his planetary model and Brahe's observations, Kepler produced in 1627 the Rudolphine Tables of planetary positions. These proved more accurate, by over an order of magnitude, than previous tables produced using the original planetary model of Copernicus . Johannes Kepler (1571-1630). Born on 27 December 1571 in Weil der Stadt, near Stuttgart, in a modest family. He graduated at age 20 from the University of Tuebingen, where he studied mathematics and astronomy under Michael Maestlin (1550-1631), an early supporter of the Copernican system. In 1594, while engaged in the final year of his studies in theology, he was given the chair of mathematics at Graz, where he became increasingly absorbed in astronomy. He was formally expelled from town in 1600 on account of his open adherence to the Protestant faith. He first came to Prague in 1599 to work as an assistant to Tycho Brahe , and upon Tycho's untimely death in 1601 inherited his massive stock of accurate planetary observations, as well as his job as Imperial Mathematician to Rudolf II. In 1612, following the downfall of Rudolf II he moved to Linz, in 1621 to Ulm, and in 1627 to Sagan. On the move again because of religious persecution, he fell ill, and died on 15 November 1630 in Regensburg. Through a quarter century of painstaking calculations Kepler brought the Copernican system to its modern form by replacing Copernicus circular heliocentric orbits by ellipses, with the Sun at one focus. The process through which he arrived at his justly famous Laws of Planetary Motion was often a contorted one, as Kepler's peculiar mixture of physical insight and mystical inclinations lead him to seek causes for the number and arrangement of planetary orbits, as opposed to constructing purely mathematical descriptions. His first such model involved the nesting of the five regular solids and was published in his 1596 Mysterium Cosmographicum . While never relinquishing this idea, in his 1619 Harmonices Mundi he also sought an explanation in terms of musical harmonies. Hidden deep in this work is the first statement of Kepler's so-called Third Law, establishing the proportionality of the square of planetary orbital periods to the cube of their mean distance to the Sun. Kepler's first two Laws of Planetary Motion were first adumbrated in his 1609 Astronomia Nova , but first laid out in detail together with his Third Law in book IV of his monumental work Epitoma astronomia Copernicanae , published between 1617 and 1621. The underlying physical explanation of his Laws would have to wait over half a century, until Isaac Newton provided the answer in terms of the theory of universal gravitation. In 1627 Kepler also finally published what was to be the crowning (but somewhat belated) achievement of Tycho Brahe's career: the Rudolphine Tables of planetary positions. These made full use of Tycho's store of accurate observations in conjunction with Kepler's new model for planetary orbits. On 28 May 1607 Kepler used his newly devised camera obscura to observe the solar disk and saw sunspot, which he mistook for a transit of Mercury, to the amazement of later astronomers who all agreed that of all people, Kepler really should have known better. Because of Kepler's position as Imperial Mathematician, his prompt and enthusiastic public endorsement of Galileo 's telescopic discoveries did a lot to publicize the latter's fame in northern Europe. Kepler was a prolific author by any standards. Besides his astronomical books, he is (by some) credited with having written the first science fiction novel, his Somnium , published posthumously in 1634 and describing a voyage to the Moon. He wrote extensively on geometrical optics, and was the first to correctly sort out once and for all the production of real versus virtual images by mirrors and lenses. He is also said to have laid the foundations of cristallography in a little book on snowflakes written as a New Years gift to his patron Rudolf II in 1611. Tycho [Tyge] Brahe (1546-1601), probably the greatest pre-telescopic astronomer. Born on 14 December 1546 in Knudstrup (Denmark) and of noble descent, he was sent by his family to study in Copenhagen, then to Leipzig to study Law, but he soon became entirely occupied with astronomy. In 1565 and 1566 Tycho studied mathematics at the universities in Wittenburg and Rostock. It is in Rostock that Tycho engaged in a duel with a fellow student and nobleman, that ended up costing him part of his nose. Tycho's reputation as an accomplished astronomer rose quickly, primarily through his observations of and writings on the 1572 Novae in Cassiopea, and of the 1577 comet . Tycho demonstrated, perhaps more convincingly than anyone before him, the falsity of the Aristotelian doctrine of the immutability of the Heavens, and of the Aristotelian theory of comets as an atmospheric phenomenon taking place in the sublunar sphere. On May 23 1576, by royal decree the Danish King Frederick II granted Tycho the island of Hven, east of Copenhagen (now part of Sweden, but a Danish possession at the time), as well as an annual stipend to further Tycho's astronomical researches. Tycho took full advantage of his independence and financial security. He established on the island the Uraniborg Observatory. Throughout his career, and in particular at Uraniborg, Tycho proceeded to build astronomical measuring instruments of unprecedented accuracy, not to mention physical size. He also established his own printing press on Hven, and build a second underground observatory with isolated observing stations to ensure reliably independent multiple astronomical measurements. Tycho was convinced of the falsity of the Ptolemaic/Aristotelian planetary model. Objecting to the motion of the Earth on physical and philosophical grounds, and unable to detect the annual parallax of the fixed stars predicted by the Copernican model, he rejected the latter as well and as a compromise proposed the Tychonian Planetary Model , in which the Earth is at the center of the universe, the Sun orbits the Earth, but all other planets orbit the Sun. From the point of view of planetary motions this yields predictions identical to those of the Copernican Model, without requiring annual stellar parallax. One of Tycho's most impressive astronomical achievement was his discovery of the Moon's so-called annual variation, a variation of the Moon's orbital speed associated with the gravitational pull of the Sun, and which shows an annual periodicity due to the slightly varying distance between the Earth and Sun in the course of the year. He also determined the length of the year to an accuracy of a few seconds. Upon losing royal support in Denmark Tycho moved to Prague and in 1598 was appointed Imperial Mathematician to the Holy Roman Emperor Rudolf II. His astronomical research program never really resumed, however. He died in Prague on October 14 1601, leaving his most recently assistant Johannes Kepler as his scientific heir.
Some large spots are visible to the unaided eye when the Sun is seen through clouds or in a camera-obscura image. But general acceptance of the reality of these apparent flaws in the Sun came only about 1611, when systematic study was begun independently by Galileo, Thomas Harriot, Johannes Fabricius, and Christoph Scheiner. S.H. Schwabe in 1843 announced discovery of the solar cycle, in which the number of spots reaches a maximum about every 11 years on the average, as do the effects of solar disturbances (e.g., auroras) on the Earth. In the first decade of the seventeenth century, four astronomers more or less simultaneously turned the newly invented telescope toward the Sun, and noted the existence of sunspots. They were Johann Goldsmid (1587-1616, a.k.a. Fabricius) in Holland, Thomas Harriot (1560-1621) in England, Galileo Galilei (1564-1642) in Italy, and the Jesuit Christoph Scheiner (1575-1650) in Germany. To Harriot belongs the oldest recorded sunspot observation, on December 8 1610, as evidenced by entries in his notebooks, but he did not pursue these observations in any systematic or continuous manner at the time. Fabricius was the first to publish his results in 1611, and correctly interpreted the apparent motion of sunspots in terms of axial rotation of the Sun. Galileo and Scheiner , however, were the most active in using sunspots to attempt to infer physical properties of the Sun. To Galileo belongs the credit of making a convincing case that sunspots are indeed features of the solar surface, as opposed to intra-Mercurial planets (Scheiner's original position). Galileo's views were first laid out in detail in his 1613 Letters on Sunspots , written in response to Scheiner own views on the matter, first published in 1612 under the pseudonym of Apelles in the form of three letters to Mark Welser (1558-1614), Augsburg Magistrate, patron of science, and scientific correspondent of both Scheiner and Galileo. Some years later Scheiner, in his massive 1630 treatise on sunspots entitled Rosa Ursina , accepted the view of sunspots as marking on the solar surface and used his accurate observations , to infer the fact that the Sun's rotation axis is inclined with respect to the ecliptic plane (i.e., the plane of the Earth's orbit around the Sun). The existence of ephemeral blemishes on the Sun's surface was in stark conflict with the then prevailing Ptolemaic/Aristotelian-based cosmology endorsed by the Roman catholic Church (after suitable modification to avoid open contradiction with the Scriptures). Galileo's views on sunspots contributed significantly the sequence of events that landed him in front of the Roman Inquisition in 1633. Officially, Galileo was condemned for disobedience to the Church, in the context of his open endorsement of the Copernican heliocentric planetary model . Growing animosity on the part of the Jesuits who, in particular through their chief astronomer Christopher Clavius (1538-1612), had been originally quite supportive of Galileo's early telescopic discoveries, also contributed to Galileo's downfall.
The Copernican system replaced the Earth by the Sun as the center of the universe, but otherwise maintained a clear distinction between the Sun, and the "fixed" stars, distributed on the fixed, outermost sphere of the copernican cosmos. This last concession to humanity's cosmic centrality was rejected by the generation of copernicans following Kepler and Galileo . Prominent among them was René Descartes (1596-1650) who, in his 1644 book Principia philosophiae , put forth a model of the cosmos where the Sun is but one of many star, each of which having formed at the center of a primeaval vortex. Descartes viewed sunspots as floating aggregates of etheral matter, accreted along the Sun's rotational axis, where centrifugal forces are negligible. The French philosopher and mathematician René Descartes was born on 31 March 1596 at La Haye, in Touraine, France. At age eight he entered the Jesuit College at La Flèche, where he developed a lasting interest in mathematics. In 1612 he began law studies at the University in Poitier, graduating four years later. He then followed along as military engineer with the army of the Prince of Nassau, returning to France in 1622. He then traveled across Europe until settling in the Netherlands in 1629. In 1649 he moved to Stockholm, Sweden, as private instructor to Queen Christina. He died there on 11 February 1650, of a severe cold his first biographers blamed on the harsh Swedish winter. In 1644 Descartes published his famed Principia Philosophiae , a work that attracted considerable interest and is said to have greatly hindered the acceptance of Isaac Newton's mechanical philosophy on the continent, and especially in France. Descartes' basic tenet was that knowledge of the universe could be obtained from a priori mathematical reasoning. Unlike his cosmological speculations, Descartes contributions to mathematics have stood the test of time. His greatest achievement his his development of analytic geometry, where geometrical objects are reduced to numbers, and algebraic techniques are used to produce mathematical counterparts to geometrical objects - for example, mathematical expressions describing curves such as parabolae, hyperbolae, and so on. Descartes was a closet Copernican who was said to have been greatly scared by the fate of Galileo , to the point of shelving a planned cosmological treatise based on the Copernican hypothesis . He did make some important contributions to physics, in particular with the co-discovery of the sine law of refraction. Sunspots observations continued in the seventeenth century, with the most active observers being the German Johannes Hevelius (1611-1687) and the French Jesuit Jean Picard (1620-1682). Very few sunspots were observed from about 1645 to 1715, and when they were their presence was noted as a noteworthy event by active astronomers. At that time, a systematic solar observing program was underway under the direction of Jean Dominique Cassini (1625-1712) at the newly founded Observatoire de Paris, with first Picard and later Philippe La Hire carrying out the bulk of the observations. Historical reconstructions of sunspot numbers indicate that the dearth of sunspots is real, rather than the consequence of a lack of diligent observers. A simultaneous decrease in auroral counts further suggest that solar activity was greatly reduced during this time period. This period is now known as the Maunder minimum , after the solar astronomer E.W. Maunder, who, following the pioneering historical investigations of Gustav Spörer (1822-1895), was most active and steadfast in investigating the dearth of sunspot sightings by astronomers active in the second half of the seventeenth century. The documented occurrence of exceptionally cold winters throughout Europe during those years may be causally related to reduced solar activity, although this remains a topic of controversy. Giovanni Domenico Cassini was born in Perinaldo in 1625, and already as a student showed great talent in mathematics and astronomy. He was trained in astronomy by two able astronomers, G.B. Riccioli and F.M. Grimaldi. Aged only 25, be was made Professor of Astronomy at the University of Bologna. Invited by the French King Louis XIV, to help in the design and construction of the forthcoming Observatoire de Paris, Cassini departed for France in 1669. Although his visit was originally intented to be limited duration, Cassini warmed up to France to the point of accepting the directorship of the Observatory in 1671, and never returned to Italy. He changed his name to Jean Dominique, and became a French citizen in 1673. He died in Paris on 14 September 1712. Cassini established his reputation in Italy by careful and accurate solar and planetary measurements, which allowed him to produce improved planetary tables, as well as improved versions of Galileo's tables for the moons of Jupiter , then of great interest because of their potential use for determining longitudes at sea. He was also involved in numerous engineering projects, including fortifications and hydraulics. While in Bologna Cassini also measured the rotation periods of Mars and Jupiter. As director of the Observatoire de Paris, Cassini inaugurated, among other projects, a sustained solar observing program. Carried out primarily by the Jesuit Jean Picard (1620-1682), and later by Philippe de La Hire (1640-1718), this program included sunspot observations as well as measurements of the solar diameter. Cassini's own interests remained in geodesy and planetary astronomy; He carried out extensive observations of Earth's Moon, and between 1671 and 1684 he discovered four Moons of Saturn (Iapetus, Rhea, Tethys and Dione). In 1675 he also discovered the main division of Saturn's ring, and correctly speculated on their physical constitution as being a collection of small debris. In 1672 Cassini effectively recomputed the size of solar system from a determination of the Martian parallax at opposition, from accurate measurements of Mars' position carried out simultaneously by himself in Paris and by fellow observer Jean Richer (1630-1696) in Cayenne, South America. Cassini's resulting value for the astronomical unit (Sun-Earth distance) was accurate to better than 90%, although the manner in which Cassini decided which of his multiple data to retain for the calculation, and which to throw out, has baffled science historians ever since. Clearly an outstanding observer, Cassini was however quite conservative on physical theories; he resisted both the Copernican model of the solar system, as well as the concept of a finite speed of light put forth by his collaborator Ole Römer (using Cassini's own data!). By 1710 Cassini had turned blind, and his younger son Jacques (1677-1756, a.k.a. "Cassini II") took over as director of the Observatoire de Paris, himself succeeded by his own son César-Francois (1714-1784, a.k.a. "Cassini III"), followed in turn by his own son Dominique (1748-1845, a.k.a. "Cassini IV"). The Observatoire de Paris thus remained under the leadership of the Cassini family for over 120 years, until Dominique resigned his position in 1793 for political reasons.
The mass of the Sun and its distance from the Earth are two very fundamental quantities that were only determined with reasonable accuracy in the eighteenth century. The first quantitative estimate of the Sun's mass is due to Isaac Newton (1642-1727). Newton presented the calculation in his Principia Mathematica , making use of his newly formulated law of universal gravitation. Newton argued that stable planetary orbits resulted from a balance between centripetal and gravitational acceleration; In doing so he could finally provide a physical explanation for the three laws of planetary motions established empirically by Kepler . The ratio of Sun-to-Earth mass can be in principle determined, without knowing the actual value of the universal gravitational constant. This only required a knowledge of orbital periods and radii. Newton, however, used too high a value for the solar parallax, thus grossly underestimating the Sun-Earth distance, and, consequently, underestimating the Sun-to-Earth mass ratio by more than a factor of ten (M Earth /M Sun =28700 instead of 332945). In later editions of his Principia (in 1713 and 1726), Newton used improved estimates of the solar parallax, and brought his estimate to within a factor of two of the modern value. Born December 25 1642 (4 January 1643 on the Gregorian calendar, then not yet adopted in England), in Woolsthorpe, Lincolnshire. Already showing signs of an exceptional intellect - and solitary personality - in grammar school, he was admitted in June 1661 to Trinity College, Cambridge, an old and prestigious institution, but somewhat in decline at the time. Although the curriculum in Trinity then focused primarily on classical Aristotelian philosophy, on his own initiative Newton became acquainted with the Descartes' writings, which introduced him to mechanistic philosophy. Between 1664 and 1666, Newton lay the groundwork of his theory of infinitesimal calculus, binomial expansion, laws of motion, theory of color, and theory of universal gravitation. He did so largely in isolation, and despite having to return to Woolsthorpe for nearly a year in the summer 1665 following the closure of Cambridge due to an outbreak of the plague. It is during these few years that nearly all of Newton's lifetime scientific achievements took shape. In October 1669 he was named Lucasian professor of mathematics, but in fact lectured very little in subsequent years. Shortly thereafter he designed and constructed the first reflecting telescope, which caused a sensation and led to his election to the Royal Society in January 1672. Newton's masterpiece, his 1687 Philosophiae Naturalis Principia Mathematica (usually refered to simply as Principia ), was composed between 1684 and 1686 and seen through publication by Edmond Halley, of cometary fame. Newton's key insight of 1666, namely that the same force of gravity felt on Earth also held the planets in their orbits around the Sun, was developed in detail therein, providing a theoretical framework for the Laws of planetary motion formulated empirically by Kepler , as well as a host of other phenomena, including the orbital paths of comets. Revised editions of the book appeared in 1713 and 1726. Newton made numerous discoveries in the field of optics, especially with respect to light and colors. His groundbreaking studies of the dispersion of light by glass prisms were begun as early as 1665, first expounded in a short essay entitled New Theory about Light and Colors published in 1672, and described in full length only much later in his 1704 Opticks . Newton's obsessive reluctance to publish his ideas or to entertain correspondence with other prominent European scientists was the chief cause of various priority disputes, the most strident and long lasting of which with his countryman Robert Hooke (1635-1703) over various matter of optics and gravitation, and with Gottfried Wilhelm Leibniz (1646-1716) over the discovery of differential calculus. From 1693 onward, Newton became increasingly involved in administrative and political affairs. In 1696 he became warden (and later master) of the Mint, a secure administrative position which provided him with a steady income. He was knighted in 1705 and presided over the Royal Society from 1703 to the end of his life. He died in Kensington on 20 March 1727, and was buried in Westminster Abbey after a state funeral.
In the 1660's Isaac Newton had shown that sunlight can be separated into separate chromatic components via refraction through a glass prism. In 1800, William Herschel extended Newton's experiment by demonstrating that invisible "rays" existed beyond the red end of the solar spectrum. He did so by detecting the temperature rise in thermometers placed beyond the red end of the visible solar spectrum. Herschel's experimental setup for the detection of invisible solar radiation. Sunlight passes through a prism (CD), forming the usual rainbow spectrum (E). A row of thermometers is positioned on a table (AB) beyond the red end of the spectrum. Thermometer 1, aligned with the spectrum, registers a rise in temperature, while the control thermometers 2 and 3 do not. Herschel boldly conjectured that these invisible caloric rays, later named infrared radiation , were fundamentally no different from visible light, and could not be seen simply because the eye is not sensitive to them. Herschell also sought caloric rays beyond the violet end of the spectrum, but to no avail. However, the following year, Johann Wilhelm Ritter (1776-1810) used an experimental setup similar to Herschel's, but placed beyond the violet end of the spectrum a piece of paper soaked in silver chloride; the subsequent blackening of the paper beyond the visible violet demonstrated the existence of ultraviolet radiation . The following year, and using similar photochemical means, William Hyde Wollaston (1766-1828) independently rediscovered ultraviolet radiation. William [Wilhelm Friedrich] Herschel was born on 15 November 1738 in Hanover, in a family of musicians. In 1757, at age 19, he fled to England. With his name anglicized to William, Herschel began earning a living as an organist and later composer and conductor. In 1772 convinced his sister Caroline to join him as an accompanying singer. In their spare time the brother-sister team became engaged in astronomy, in particular in the building of ever greater size and magnifying power. William died at his home in Slough, near Windsor, on August 25 1822, and Caroline on September 1 1848. Herschel's first major discoveries were to show that Mars and Jupiter exhibit axial rotation. Herschel struck fame in 1781, when on March 13 he discovered the planet Uranus while engaged in work aimed at determining stellar parallax. This being the first new planet discovered since Antiquity, Herschel, until then a mere amateur astronomer relatively unknown even in England, became a celebrity. Adopting an historically proven strategy, Herschel named the new planet Georgium Sidum , in honor of the then ruling English monarch George III. The trick worked once again, as King George III gave William and Caroline the titles of "The King's Astronomer" and "Assistant to the King's Astronomer", an honor which came with a life's pension for both. In 1782 they moved to Bath, and shortly thereafter to Slough, near Windsor, and from this point on William and Caroline could devote themselves entirely to astronomy. The Heschels went on to discover two moons of Uranus in 1787, and two new moons of Saturn. Herschel's work on double stars, which he pursued intermittently between 1782 and 1821, provided the first demonstration that gravity acted also outside the solar system. He also made one of the first attempts at measuring the Sun's motion through the galaxy via the proper motion of nearby stars. This became yet another step in the removal from the Earth as the center of the Universe. His increasingly large and powerful telescopes also allowed him to resolve many of the hitherto mysterious "nebulae" into clusters of faint stars. Anticipating in some ways Laplace's model for the formation of the solar system, Herschel developed an evolutionary theory of the universe, where, starting from a uniform "initial state", stars form and clump into nebulae. While Caroline became increasingly occuppied with the search for comets (at which she was quite successful), William became for a time interested in the Sun. Inspired by Wilson's 1774 work, he put forth the theory of sunspots as openings in the Sun's luminous atmosphere, an opinion that was to endure well into the nineteenth century. In 1800 he became interested in the solar spectrum, and uncovered the first evidence for solar energy output outside of the visible spectrum , in what is now known as the infrared. In 1801 he published two papers that, in part because of Herschel's reputation, effectively launched the field of solar influences on Earth's weather. It is in the first of these papers that Herschel discusses an anticorrelation between the price of wheat and the number of sunspots visible on the Sun.
Wollaston's experimental setup for the prismatic observation of the solar spectrum. Wollaston believed that the lines labeled here B, C and E marked natural color boundaries, although he also noticed other dark lines (f,g) that did not appear to delineate colors. Reproduced from Philosophical Transactions of the Royal Society of London , vol. 92 (1802), p. 380 (Plate XIV). While investigating the refractive properties of various transparent substances, the English chemist and physicist William Hyde Wollaston (1766-1828) noticed dark lines in the spectrum of the Sun, as viewed through a glass prism following the method of Isaac Newton . Beyond suggesting that these dark lines marked the boundaries of "natural colors", Wollaston did not pursue the matter much further. Yet this marked the first step towards solar spectroscopy, which was to revolutionalize Solar Physics in the second half of the century. In what was to later lead to some of the more important advances in solar physics, Joseph von Fraunhofer (1787-1826) independently rediscovered the 'dark lines' in the solar spectrum noticed 15 years earlier by William Hyde Wollaston (1766-1828). Fraunhofer pursued the matter mainly because he saw the possibility of using the lines as wavelength standards to be used to determine the index of refraction of optical glasses. Other physicists, however, were quick to realize that the Fraunhofer lines could be used to infer properties of the solar atmosphere, as similar lines were being observed in the laboratory in the spectrum of white light passing through heated gases. Reproduction of Fraunhofer's original 1817 drawing of the solar spectrum. The more prominent dark lines are labeled alphabetically; some of this nomenclature has survived to this day [from: Denkschriften der K. Acad. der Wissenschaften zu München 1814-15, pp. 193-226]. Compare this to Wollaston's drawing . In the hands of David Brewster (1781-1868), Gustav Kirchhoff (1824-1887), Robert Wilhelm Bunsen (1811-1899), and Anders Jonas Ångström (1814-1874), to name but a few, spectroscopy turned into a true science which revolutionized not only solar physics, but also astronomy at large. Still today, most information gathered on the Sun and stars is obtained through spectroscopic means. Born in Straubing, Bavaria, on 6 March 1787. Orphaned at the age of 11, he did an apprenticeship as lens- and mirror-maker, after which he became employed in a Munich company making scientific instruments. He learned mathematics and became very skilled in applied optics. He was active in the Bavarian academy of sciences, where his skills in applied optics attracted attention. In 1823 he was appointed director of the Physics Museum in Munich, and received the (honorary) title of Professor. He died of tuberculosis in Munich on 7 June 1826. In 1812-1814 Fraunhofer was fully engaged in the design of achromatic objective lenses for telescopes, which required the accurate determination of refractive indices of optical glasses. Examining the spectrum of solar light passing through a thin slit, he noticed a multitude of dark lines. These had not been noticed by Newton in 1666 because he shone light from the whole solar disk through a circular hole into his prisms. They had been noticed by William H. Wollaston in 1802, who however recorded the presence of only seven dark lines. Well aware that some of these lines could be used as wavelength standards, Fraunhofer accurately measured the position of 324 of the 500 or so lines he could see in his solar spectrum. He labeled the most prominent spectral lines with letters, establishing a nomenclature that survives to this day. Although he did notice and comment upon the wavelength coincidence between his D line and a prominent line in laboratory flame spectrum (now known to be due to Sodium), he did not pursue the matter much further. In 1821 Fraunhofer built the first diffraction grating, comprised of 260 close parallel wires. Well versed in the mathematical wave theory of light, Fraunhofer used his diffraction grating to actually measure wavelength of specific colors and dark lines in the solar spectrum. He also built and studied reflection gratings. Fraunhofer did succeed, in 1817, in designing an achromatic objective lense; with minor modifications, his design is still in use today.
In the late 1850s the chemist Robert Wilhelm Bunsen (1811-1899) and theoretical physicist Gustav Kirchhoff (1824-1887), both at Heidelberg, took on the issue of spectral line identification pretty much where Fraunhofer had left it some 40 years earlier. By simultaneous observations of the solar spectrum and laboratory flame spectra, they showed that (bright) emission lines in heated gases coincide with (dark) absorption lines seen when observing white light shining through the same cool gas. This established the empirical basis needed for the identification of the dark lines seen in the solar spectrum. By careful comparison with emission lines seen in the laboratory for various pure gases, Kirchhoff could demonstrate the existence in the Sun of a large number of chemical elements, mostly metals, also present on Earth. Hydrogen was identified spectroscopically in 1862 by A. Ångström (1814-1874), but it is only much later, in the 1920's, that Hydrogen was recognized as the most abundant solar constituent. Reproduction of part of the map of the solar spectrum published in 1863 by Kirchhoff, showing the identification of a large number of spectral lines with various chemical elements. Note numerous clear matches for Iron (Fe). Following this and other groundbreaking work by David Brewster (1781-1868) and Ångström, spectroscopy continued to progress throughout the second half of the eighteenth century. In the solar context, some of the most active and innovative observers were J. Norman Lockyer (1836-1920) Jules Janssen (1824-1907), Hermann Carl Vogel (1841-1907), William Huggins (1824-1910), Angelo Secchi (1818-1878), Charles Young (1834-1908), and Samuel Langley (1834-1906). Even at that time, spectroscopy was still an empirical science without a sound physical basis, as quantum mechanics lay half a century in the future.
Early sunspots observers noted the curious fact that sunspots rarely appear outside of a latitudinal band of about ± 30° centered about the solar equator, but otherwise failed to discover any clear pattern in the appearance and disappearance of sunspots. In 1826, the German amateur astronomer Samuel Heinrich Schwabe (1789-1875), set himself about the task of discovering intra-mercurial planets, whose existence had been conjectured for centuries. Like many before him, Schwabe realized that his best chances of detecting such planets lay with the observation of the apparent shadows that they would cast upon crossing the visible solar disk during conjunction; the primary difficulty with this research program was the ever-present danger of confusing such planets with small sunspots. Accordingly, Schwabe began recording very meticulously the position of any sunspot visible on the solar disk on any day that weather would permit solar observation. In 1843, after 17 years of observations, Schwabe had not found a single intra-mercurial planet, but had discovered something else of great importance: the cyclic increase and decrease with time of the average number of sunspot visible on the Sun, with a period that Schwabe originally estimated to be 10 years. Variation in observed sunspot numbers during the time period 1800-present. The red curve is the Wolf sunspot number, and the purple line a count of sunspot groups based on a reconstruction by D.V. Hoyt. The green crosses are auroral counts, based on a reconstruction by K. Krivsky and J.P. Legrand. Samuel Heinrich Schwabe was born on 25 October 1789 in Dessau, near Berlin. He began pharmaceutical studies in Berlin, in the course of which he became interested in astronomy and botany. He returned to Dessau in 1812 to take over his family's pharmacy, while pursuing astronomical and botanical researches as an amateur. His first telescope was won at a lottery in 1825, but the following year he ordered a more powerful one through Fraunhofer . Becoming increasingly absorbed his astronomical studies, he sold the family business in 1829. Schwabe died in Dessau on 11 April 1875. Schwabe observational work was aimed originally at discovering possible intramercurial planets. Starting on October 11 1825, he observed the Sun virtually every day that the weather allowed, and did so continuously for 42 years. In doing so he accumulated volumes of sunspot drawings, the idea being to detect his hypothetical planet as it passed across the solar disk, while avoiding confusion with small sunspots. In 1843 Schwabe still had not discovered any new planet, but instead his 17 years of nearly continuous sunspot observations revealed a 10-year periodicity in the number of sunspots visible on the solar disk. That same year Schwabe published this interesting result in the Journal Astronomische Nachrichten , but it attracted little attention until 1851 when his sunspot data was included by Alexander von Humboldt (1769-1859) in volume III of his monumental Kosmos . Curiously, Schwabe's astronomical researches initially won him greater recognition in England than in Germany. In February 1857 he was awarded the Gold Medal of the Royal Astronomical Society, and in 1868 he was elected to the Royal Society. While Schwabe's fame as an astronomer rests chiefly on his discovery of the sunspot cycle, he is also credited with the first description and drawing, in 1831, of Jupiter's great red spot.
The first photographic technique was developed in the 1830's by J. N. Niepce (1765-1833) and Louis Daguerre (1789-1851), and relied on the exposure of a thin iodine layer deposited on a silver substrate, subsequently fixed in a mercury bath. The images so produced became known as daguerrotypes . This imaging technique was very soon applied to astronomy, through the enthusiastic support of the French astronomer and politician Francois Arago (1786-1853), and the British astronomer John Herschel (1792-1871, son of William Herschel ), who first coined the term "photography", as well as "positive" and "negative" images. The first successful daguerrotype of the Sun, reproduced below, was made on 2 April 1845 by the French physicists Louis Fizeau (1819-1896) and Léon Foucault (1819-1868) (the two being perhaps better known for their various pioneering measurements of the speed of light). The exposure was 1/60 of a second. This image shows the umbra/penumbra structure of sunspots, as well as limb darkening. Reproduction of the first daguerrotype of the Sun. The original image was a little over 12 centimeters in diameter. Reproduced from G. De Vaucouleurs, Astronomical Photography , MacMillan, 1961. Daguerre's photographic process was soon supplanted by a new technique developed starting in 1851, based on a colloidal suspension on a glass substrate, in essence the direct ancestor of modern photographic film. In 1858 daily photographic record of the solar disk using a solar telescope especially designed for photography began at Kew, in England, under the leadership of Warren De la Rue (1815-1889). Photographic techniques were soon thereafter applied to the study of prominences, solar granulation, and solar spectroscopy, with some of the more spectacular results of the period obtained by Jules Janssen (1824-1907) at Meudon, near Paris. The first photograph of a solar prominence was captured by Charles A. Young (1834-1908) in 1870. The first useful Daguerrotype of a solar eclipse was secured on 28 July 1851 by the photographer/astronomer Berkowski at the Königsberg observatory (then in Prussia, now Kalinigrad in Russia). De la Rue's group also obtained many fine photographs of the 18 July 1860 total eclipse in Spain. Eclipse photographic techniques were further improved by the introduction of radial gradient filters, designed to differentially attenuate the innermost, brightest portion of the corona. The resulting photographs allow to discern details of coronal structure out to many solar radii.
As Schwabe's discovery of the sunspot cycle gained recognition, the question immediately arose as to whether the cycle could be traced farther in the past on the basis of extant sunspot observations. In this endeavour the most active researcher was without doubt the Swiss astronomer Rudolf Wolf (1816-1893). Faced with the daunting task of comparing sunspot observations carried out by many different astronomers using various instruments and observing techniques, Wolf defined the relative sunspot number ( r ) as follows: r=k(f+10g) where g is the number of sunspots groups visible on the solar disk, f is the number of individual sunspots (including those distinguishable within groups), and k is a correction factor that varies from one observer to the next (with k=1 for Wolf's own observations, by definition). This definition is still in used today, but r is now usually called the Wolf (or Zürich) sunspot number. Wolf succeeded in reliably reconstructing the variations in sunspot number as far as the The 1755-1766 cycle, which has has since been known conventionally as "Cycle 1", with all subsequent cycles numbered consecutively thereafter; at this writing (January 2000), we are in the rising phase of cycle 23.
By observing spots, R.C. Carrington found (c. 1860) that the Sun rotates not as a solid body but differentially, fastest at the equator. Sunspots are never seen exactly at the equator or near the poles. Vilhelm Bjerknes theorized in 1926 that spots are the erupting ends of magnetic vortices broken by the Sun's differential rotation. Various elaborations on this idea have been proposed, but the cause of sunspots is still uncertain. George Ellery Hale in 1908 discovered their magnetic fields; John Evershed in 1909 detected the radial motion of gas away from sunspot centres. Annie Russel Maunder in 1922 charted the latitude drift of spots during each solar cycle. Her chart is sometimes called the butterfly diagram because of the winglike shapes assumed by the graph. Each solar cycle begins with small spots appearing in middle latitudes of the Sun. Succeeding spots appear progressively closer to the Sun's equator as the cycle reaches its maximum level of activity and declines. Early nineteenth century solar astronomers were increasingly intrigued at the fact that determinations of the solar rotation period obtained by tracking sunspots carried out over the preceding two centuries varied between anywhere from 25 to 28 days. This difference, while small, was significantly larger than the accuracy with which good observers could track sunspot motion. The resolution of this puzzle came in 1858, when Richard C. Carrington (1826-1875) in England and shortly thereafter Gustav Spörer (1822-1895) in Germany independently made two key discoveries. First, the latitude at which sunspots are most often seen decreases systematically from about 40° to 5° latitude as the sunspot cycle proceeds from one minimum to the next (see diagram below). Second, sunspots located at higher latitudes are carried around the sun more slowly than spots at lower latitudes. From this, Carrington concluded that the Sun rotates differentially , yet another argument in favor of the fluid or gaseous nature of the Sun's outer layers. The aforementioned historical discrepancies are then explained by the fact that different astronomers simply observed the Sun at different epochs of the cycle. Spörer's Law of sunspot migration. The thick lines shows the latitude] at which most sunspots are found (vertical axis, equator is at zero), as a function of time (horizontal axis). The dashed line is the Wolf sunspot number, showing the rise and fall of the solar cycle. The rapid development of spectroscopic techniques in the second half of the nineteenth century offered another mean of measuring the surface differential rotation, one moreover that is not restricted to latitudes where sunspots are present: measurement of the wavelength shift of spectral lines between the approaching receding solar limb, as a consequence of the Doppler effect. This was first carried out by Hermann Vogel (1841-1907) in 1871, and a few years after by Charles Young (1834-1908). These results were accurate enough to demonstrate that sunspots rotate at very nearly the same rate as the sun's photosphere. By the late 1880's Nils Dúner (1839-1914) had secured accurate spectroscopic rotational period determinations at latitudes about twice higher than the sunspot belts, demonstrating that the Sun's polar regions rotate about 30% more slowly than its equator. Interestingly, Christoph Scheiner had already noted in his 1630 Rosa Ursina that the rotation period inferred from tacking sunspots at different heliocentric latitudes showed a systematic increase with latitude. However, in Scheiner's Aristotelian framework the Sun could only be a solid, rigidly rotating sphere, and therefore he interpreted his data a proof that sunspots were not markings on the solar surface, but instead cloud-like structures floating above it, since a fluid Sun was "physically absurb". For this reason, most historians of science continue to attribute the discovery of solar differential rotation to Carrington and Spörer.
On 1 September 1859, the amateur astronomer Richard C. Carrington (1826-1875) was engaged in his daily monitoring of sunspots, when he noticed two rapidly brightening patches of light near the middle of a sunspot group he was studying (indicated by A and B on the drawing below). In the following minutes the patches dimmed again while moving with respect to the active region, finally disappearing at positions C and D. This unusual event was also independently observed by R. Hodgson (1804-1872), another British astronomer. Reproduction of a drawing by R.C. Carrington, showing the location of the flare he observed while making a drawing of an active region. Reproduced from his 1860 paper in Monthly Notices of the Royal Astronomical Society (vol. 20 , p. 13). This serendipitous observation represents the first clear description of a solar flare , corresponding to a sudden and intense heating of solar atmospheric plasma caused by reconnection of magnetic fields. What Carrington observed would today be called a two-ribbon flare. Only the largest flares are bright enough to be seen in visible light. They are readily seen in X-rays, however. An earlier, plausible observational report of a white light flare has been found in the (unpublished) notebooks of the English scientist and amateur astronomer Stephen Gray (1666-1736), who on 27 December 1705 observed what he described as a ``flash of lightning'' near a sunspot. Both Carrington and Hodgson noted that magnetic monitoring instruments registered strong disturbances at about the same time, but it is not possible to tell for sure whether these were due to the flare they actually saw. It is more likely that they were caused by other generalized solar disturbances of which the flare was but one manifestation.
The total solar eclipse of 18 July 1860 was probably the most thoroughly observed eclipse up to that time. The three drawings are a sample of drawings produced at that time which include depictions of a peculiar feature in the SW (lower right) portion of the corona. Based on comparison with modern coronal observations, it is quite likely that these represent the first record of a Coronal Mass Ejection in progress. Diagrams Drawings of the 1860 eclipse by G. Tempel (top left), von Feilitzsch (top center), F.A. Oom (top right), E.W. Murray (bottom right), F. Galton (bottom center), and C. von Wallenberg (bottom right). Reproduced from Ranyard, C.A 1879, Mem. Roy. Astron. Soc., 41 , 520, chap. 44. Today coronal mass ejections are known to represent one of the more energetic - and geoeffective - manifestation of solar activity, with up to 10 billion tons of material being ejected into interplanetary space at speeds reaching up to 1000 kilometer per second.
The study of sunspots and their 11-year cycle was finally put on a firm physical footing by the epoch-making work of George Ellery Hale (1868-1938) and collaborators, in the opening decades of the twentieth century. In 1907-1908, by measuring the Zeeman splitting in magnetically sensitive lines in the spectra of sunspots and detecting the polarization of the split spectral components, Hale provided the first unambiguous and quantitative demonstration that sunspots are the seats of strong magnetic fields . Not only was this the first detection of a magnetic field outside the Earth, but the inferred magnetic field strength, 3000 Gauss, turned out over a thousand times greater than the Earth's own magnetic field. It was subsequently realized that the pressure provided by such strong magnetic field would also lead naturally to the lower temperatures observed within the sunspots, as compared to the photosphere. George Ellery Hale was born in Chicago on 29 June 1868. A single child heir to his family's considerable fortune, Hale developed an interest in astronomy at a young age. In this he benefited from the continuing moral and financial support of his father, who over the years of his childhood and teenage years purchased him telescopes and spectrometers of increasing power. By 1891 Hale was effectively equipped with his own private solar astronomical laboratory. This unusual opportunity was not lost on Hale, who went on to become one of the foremost astronomer in the U.S. In 1890 he graduated from the Massachusset Institute of Technology with a bachelor's degree in Physics, at which time his scientific reputation was already well established. Although he never completed a doctorate, in 1892 he was appointed professor of Astronomy at the University of Chicago (without salary for the first three years, however), and launched in a life long campaign to fund and build ever better astronomical observatories. Overworked and suffering from recurrent episodes of depression, Hale resigned as director of Mt. Wilson Observatory in 1923, and retired from the active scientific research scene in the following years, arguably at the height of his career. He died on 21 February 1938 in Pasadena, California. Even though Hale capitalized heavily on his family's wealth and various connections in the mid-west's financial circles, his talents and energy as an organizer and fund raiser on behalf of various astronomical projects remains extraordinary (and arguably as yet unsurpassed) by any standards. Over the years he organized the founding of three world class astronomical observatories; in the 1890s he secured funding for the establishment of the University of Chicago's Yerkes Observatory, in nearby William's Bay, Wisconsin. That observatory became fully operational in 1897, and harbored for a time the largest telescope in the world. He then secured funds for the establishment of a solar observatory on Mt. Wilson in California, of which he became director in 1904, and which long remained the best solar observatory in the world, in addition to hosting for a time the world's largest night time telescope. Though he did not live to see the project taken to completion in 1948, Hale was the main force behind the construction of the 5 meter telescope on Mt. Palomar, which remained for over three decades the the world's largest optical telescope. He was also instrumental in the founding of the American Astronomical Society in 1899, and later in turning the then relatively unknown Throop Polytechnical Institute in Pasadena, into what is now the California Institute of Technology. In 1895 Hale co-founded (and edited for nearly 30 years) The Astrophysical Journal . This was originally envisioned as an international forum for the publication of astronomically relevant papers in the field of spectroscopy, but the journal rapidly expanded its scope to become (and remains to this day) the world's leading research Journal in the field of Astrophysics. In the first two decades of the twentieth century, Hale and his collaborators were constantly innovating and pushing the limit of astronomical and spectroscopic instrumentation. They effectively invented specialized solar tower telescopes, and pioneered the field of spectropolarimetry. Hale's most acclaimed scientific work was his demonstration that sunspots are the seat of strong magnetic fields, and that their polarity reveals striking spatial and temporal regularities that betray the presence of a well-organized, large-scale magnetic field in the solar interior. Nearly a century after Schwabe's discovery of the 11 year sunspot cycle, Hale's work on sunspots finally put solar cycle studies on a truly physical footing.
