Astronomy by Victor R. OribePresentation Transcript
Ancient Astronomy Astronomy probably began long before recorded
history (more than 5,000 years ago) when human began to track the
motion of celestial objects so they knew when to plant their crops or
prepare to hunt migrating herd. The ancient Chinese, Egyptians, and
Babylonians are well known for their record keeping. These cultures
recorded the location of the Sun, Moon, and the five visible planets as
these objects moved slowly against the background of “fixed” stars. It was
not enough to track the motions of celestial objects; predicting their future
positions (to avoid getting married at an unfavorable time, for example)
A study of Chinese archives shows that the Chinese recorded every
appearance of the famous Halley’s Comet for at least 10 centuries. Like
most ancients, the Chinese considered comets to be mystical, generally
comet were seen as bad omen and were blamed for a variety of disaster,
from wars to plague. The Chinese has an accurate records of “guest stars”.
Today we know that a “guest star” is a normal star, usually too faint to be
visible, which increases its brightness as it explosively ejects gases from its
surface, a phenomena we call a Nova or Supernova.
The Golden Age of Astronomy The Golden Age of early astronomy (600
B.C. – AD 150) was centered in Greece. The basics of Geometry and
Trigonometry, which they developed, were used to measure the size of and
distances to the largest-appearing bodies in the heavens –Sun and the
The Golden Age of early astronomy (600 B.C. –AD 150) was centered in
The early Greeks held the incorrect Geocentric (geo = Earth, centric =
centered) view of the universe. Orbiting the Earth were the moon, Sun, and
known planets – Mercury, Venus, Mar s, Jupiter and Saturn Beyond the
planets was a transparent, hallow celestial sphere on which stars were
attached and travelled around the Earth. The sun and the moon were
thought to be perfect crystal sphere
The famous Greek Philosopher Aristotle (384-322BC) concluded that Earth
is spherical because it always casts a curved shadow when it eclipses the
moon. Although most of Aristotle’s teaching were considered infallible by
may for centuries after his death, his belief in a spherical Earth was lost
during the Middle Ages.
Measuring the Earth’s Circumference The first successful attempt to
establish the size of the Earth was credited to Eratosthenes (276-194 B.C.)
Eratosthenes observed the angles of the noonday Sun in two Egyptian
cities that were roughly north and south of each other- Syene (now Aswan)
Finding the angles of the noonday sun differed by 7 degrees, or 1/50 of a
complete circle, he concluded that the circumference of Earth must be 50
times the distance between these two cities. The cities were 5,000 stadia
apart, giving him a measurement of 250,000 stadia. Many historians
believe the stadia was 157.6 meters, which would make Eratosthenes’s
calculation of Earth’s circumference – 39,400 km.- a measurement very
close to the modern value of 40. 0705 km.
The Sun-Centered Universe The first Greek to profess a Sun-Centered, or
Heliocentric , (helios=sun, centric=centered) universe was Aristarchus
(312- 230 B.C.) Because of the strong influence of Aristotle’s writing, the
Earth-centered view dominated Western thought for nearly 2,000 years.
Mapping the Stars The greatest among Greek astronomers was
Hipparchus, best known for his star catalogue. Hipparchus determined the
location of almost 850 stars, which he divided into six groups according to
their brightness (the system is still used today) He measured the length of
the year to within minutes of the modern value and developed a method for
predicting the times of lunar eclipse to within a few hours.
Many of the Greek discoveries were lost during the Middle Ages, the Earth-
centered view that the Greek proposed became entrenched in Europe.
Claudius Ptolemy Much of our knowledge Of Greek astronomy comes from
a 13-volume treatise, Almagest (the great work) which was compiled by
Ptolemy in A.D. 141. With the decline of the Roman Empire around fourth
century, much of the accumulated knowledge disappeared as libraries
were destroyed After the decline of Greek and Roman civilization, the
center of astronomical study move east to Baghdad, where fortunately,
Ptolemy’s work was translated into Arabic
Arabic astronomers expanded Hipparchus’s star model catalog and divided
the sky into 48 constellations – the foundation of our present-day
constellation system. It wasn’t until some time after the tenth century that
the ancient Greeks’ contributions o astronomy were reintroduced to Europe
through the Arabic community. The Ptolemaic model soon dominated
European thought as the correct representation of the heavens, which
created problems for anyone who found errors in it.
The Birth of Modern Astronomy Ptolemy’s Earth-centered universe was not
discarded overnight. Modern astronomy’s development was more than
scientific endeavor, it require a break from deeply entrenched philosophical
and religious views that had been a basic part of Western society for
thousands of years. The work of five noted scientists undergo a transition
from an astronomy that merely describe what is observed, to an astronomy
that tries to explain what is observed and more importantly why the
universe behaves the way it does.
Nicolaus Copernicus For almost 13 centuries after the time of Ptolemy,
very few astronomical advances were made in Europe, some were even
lost, including the notion of a spherical Earth. The first great astronomer to
emerge after the Middle Ages was Nicolaus Copernicus (1473-1543) from
Poland After discovering Aristarchus’s writing, Copernicus became
convinced that Earth is a planet, just the other five then-known planet. The
daily motions of the heavens, he reasoned could be more simply explained
by rotating Earth.
Having concluded that the Earth is a planet, Copernicus constructed the
heliocentric model for the solar system with the Sun at the center and the
planets Mercury, Venus, Earth, Mars, Jupiter, and Saturn orbiting it. This
was a major break from the ancient and prevailing idea that a motionless
Earth lies at the center of all movement in the universe. At that this time it
was considered heretical b many Europeans. Anyone who refuse to
denounce the Copernican theory were burned at the stake.
Tycho Brahe Tycho Brahe was born of Danish nobility three years after the
death of Copernicus. He persuaded King Frederick II to establish an
observatory near Copenhagen, which headed. There he designed and built
pointers (telescope would not be invented for a few more decades), which
he used for 20 years to systematically measure the location of the
heavenly bodies in an effort to disprove the Copernican theory
His observations, particularly of Mars, were far more precise than any
made previously and are his legacy to astronomy. With the death of his
patron, the King of Denmark, Tycho was forced to leave his observatory.
Tycho moved to Prague in the present day Czech Republic, where, in the
last year of his life, he acquired an able assistant, Johannes Kepler. Kepler
retained most of the observations made by Tycho and put them to
exceptional use. Ironically, the data Tycho collected to refute the
Copernican view of the solar system would later be used by Kepler to
Johannes Kepler Armed with Tycho’s data, a good mathematical mind,
and, of greater importance, a strong belief in the accuracy of Tycho’s work,
Kepler derived three basic laws of planetary motion 1. The path of each
planet around the Sun, while almost circular, is actually an ellipse, with the
sun at one focus. This law allow us to calculate astronomical events like
eclipses, comets, spacecraft rendezvous, and satellite action.
2. Each planet revolves so that an imaginary line connecting it to the Sun
sweeps over equal areas in equal intervals of time. In order for a planet to
sweep equal areas in the same amount of time, it must travel more rapidly
when it is nearer the Sun (perihelion) and more slowly when it is farther
from the sun (aphelion).
Kepler was devout and believed that the Creator made an orderly universe
and that this order would be reflected in the positions and motions of the
planets. The uniformity he tried to find eluded him for nearly a decade.
Then in 1619, Kepler published his third law in the Harmony of the Worlds
3. The planet orbital period squared is equal to its mean solar distance
cubed. The solar distances of the planets can be calculated when their
periods of revolutions are known. For example: Mars has an orbital period
of 1.88 years, which squared equals 3.54. The cube root of 3.54 is 1.52,
and that is the average distance from Mars to the Sun in astronomical unit.
Kepler’s law assert that the planets revolve around the Sun, and therefore
support the Copernican theory.
Galileo Galilei He was the greatest Italian scientists of the Renaissance.
He was a contemporary of Kepler, and like Kepler, strongly supported the
Copernican theory of a Sun-centered solar system. Galileo’s greatest
contributions to science were his descriptions of the behavior of moving
objects, which he derived from experimentation. All astronomical
discoveries before Galileo’s time were made without the aid of a telescope.
In 1609, Galileo heard that a Dutch lens maker had devised a system of
lenses that magnified objects. Apparently without ever seeing the
telescope, Galileo constructed his own, which magnified distant object
three times the size seen by the unaided eye. He immediately made
others, the best having magnification of about 30. With the telescope,
Galileo was able to view the universe in a new way.
Galileo made many discoveries that supported the Copernican view of the
Universe including the following: 1. The discovery of Jupiter’s four largest
satellite or moons. This find dispelled the old idea that Earth was the sole
center of motion in the universe; for here, plainly visible, was another
center of motion –Jupiter.
2. The discovery that the planets are circular disk rather than just points of
light, as was previously thought. This indicated that the planets must be
Earth-like revolved around the sun.
3. The discovery that Venus exhibits phases just as the Moon does and
that Venus appears smallest when it is in full phase and thus is farthest
from Earth This observation demonstrates that Venus orbits its source of
light –the Sun. In the Ptolemaic system, the orbit of Venus lies between
Earth and the Sun, which means that only the crescent phases of Venus
should ever be seen from Earth.
4. The discovery that Moon’s surface is not a smooth glass sphere, as the
ancient had proclaimed.
Galileo saw mountains, craters, and plains indicating that the Moon was
Earth-like. He thought that plains might be bodies of water, and this idea
was strongly promoted by others, as we tell from the names given to these
features (sea of tranquility, sea of storms, etc.
5. The discovery that the sun (the viewing of which may have caused the
eye damage that later blinded him) has sunspot – dark regions caused by
slightly lower temperatures. Galileo tracked the movement of these spots
and estimated that rotational period of the Sun as just under a month.
Hence, another heavenly body was found to have both “blemishes” and
In 1616, the Church condemned the Copernican Theory as contrary to
Scripture because it did not put humans as their rightful place at the center
of Creation, and Galileo was told to abandon this theory. Undeterred,
Galileo began writing his famous work, Dialogue of the Great World
Systems. Despite poor health, he completed the project and in 1630 went
to Rome, seeking permission from Pope Urban VIII to publish. Since the
book was a dialogue that expounded both the Ptolemaic and Copernican
system, publication was allowed. However, Galileo’s detractors were quick
to realize that he was promoting Copernican view at the expense of the
The sale of the book was quickly halted, and Galileo was called before the
Inquisition. Tried and convicted of proclaiming doctrines contrary to
religious teaching, he was sentenced to permanently house arrest, under
which he remained for the last 10 years of his life. Despite this restriction,
and his grief following the death of his eldest daughter, Galileo continued to
work. In 1637 he became totally blind, yet during the next few years he
completed his finest scientific work, a book on the study of motion in which
he stated that the natural tendency of an object in motion is to remain in
motion. Later, as more scientific evidence in support of the Copernican
system was discovered, the Church allowed Galileo’s works to be
Sir Isaac Newton Sir Isaac Newton (1642- 1727) was born in the year of
Galileo’s death. His many accomplishments in Mathematics and Physics
led a successor to say that “Newton was the greatest genius that ever
At the age of 23, he envisioned a force that extends from Earth into space
and holds the Moon in orbit around Earth. He was the first to formulate and
test the Law of Universal Gravitation. It states that: “Every body in the
universe attracts every other body with a force that is directly proportional
to their masses and inversely proportional to the square of the distance
The Law of gravitation also states that the greater the mass of an object,
the greater its gravitational force.
Constellations The division of the sky into areas. Constellation (con = with,
stella = star) Usually named in honor of mythological character or great
heroes. Sometimes it takes a bit of imaginations to make out the intended
subjects, as most constellation were probably not thought of as likeness in
the first place.
Summer Constellation in the Northern Hemisphere
The Big Dipper
Although the stars that make up constellation all appear to be the same
distance from Earth, this is not the case. Some are many times farther
away than others. Thus, stars in a particular constellation are not
associated with each other in any important physical way. Various cultural
groups, including Native Americans and the Chinese, attached their
names, pictures, and stories to the constellations. For example, the
constellation Orion the hunter was known as the White Tiger to ancient
Astronomers use constellations when they want to roughly identify the area
of the heaven they are observing. Some of the brightness stars in the
heavens were given proper names such as Sirius, Arcturus, and
Betelgeuse. The brightness stars in a constellation are generally names in
order of their brightness by the letter of the Greek alphabet –alpha (α), beta
(β), and so on – followed by the name of the parent constellation. For
example, Sirius, the brightness star in the constellation Canis Major (larger
Dog) is also called Alpha (α) Canis Majoris.
Motion of Earth 1. Rotation The main consequences of Earth’s rotation are
day and night. Earth’s rotation has become a standard method of
measuring time because it is so dependable and easy to use.
Earth’s rotation is measured in two ways, making two kinds of days. 1)
Mean Solar Day The time interval from one noon to the next, which
averages about 24 hours. Noon is when the Sun has reached its zenith
(highest point in the sky).
2) Sidereal (sider=star, at=pertaining to) day Is the time it takes for Earth to
make one complete rotation (3600) with respect to a star other than our
sun. The sidereal day is measured by the time required for a star to
reappear at the identical position in the sky. The sidereal day has a period
of 23 hours, 56 minutes, and four seconds, which is almost four minutes
shorter than the mean solar day. This difference results because the
directions to distant stars changes only infinitesimally, whereas the
direction to the Sun changes by almost 1 degree each day.
2. Revolution Earth revolves around the Sun in an elliptical orbit at an
average speed of 107, 000 km per hour. Its average distance from the sun
is 150 million km.
Because of its elliptical orbit. Earth’s distance from the sun varies.
Earth’s axis is tilted about 23.50. This angle is very important to Earth’s
inhabitants because the inclination of Earth’s axis causes the yearly cycle
3. Precession A very slow movement of the Earth is called axial
Although Earth’s axis maintains approximately the same angle of tilt, the
direction in which the axis point continually changes. As a result, the axis
traces a circle in the sky.
At present time, the axis points toward the bright star Polaris. In AD
14,000, it will point toward the bright star Vega, which will then be the North
Star for about a thousand years or so. The period of precession is 26,000
years. By the year 28,000, Polaris will once again be the North Star.
Precession has only a minor effect on the season because Earth’s angle of
tilt changes slightly.
Motion of the Earth-Moon System Earth has one natural satellite, the
In addition to accompanying Earth in its annual trek around the Sun, our
Moons orbits Earth about once each month.
When viewed from a Northern Hemisphere perspective, the Moon moves
counterclockwise (eastward) around the earth.
The Moon’s orbit is elliptical, causing the Earth-Moon distance to vary by
about 6 percent, averaging 384,401 km.
Lunar Motion The cycle of the Moon through its phases requires 29 ½ days
– a time span called the synodic month. This cycle was the basis for the
first Roman calendar
Sidereal Month is the apparent period of the Moon’s revolution around the
Earth and not the true period, which takes only 27 1/3 days.
As the moon orbits Earth, the Earth-Moon system also moves in an orbit
around the Sun. Consequently, even after the Moon has made a complete
revolution around Earth, it has not yet reached its starting position with
respect to the Sun, which is directly between the Sun and the Earth.
An interesting fact concerning the motions of the Moon is that its period of
rotation around its axis and its revolution around the Earth are the same -
27 1/3 days. Because of this, the same lunar hemisphere always faces
Earth. All of the landings of the manned Apollo missions were confined to
the Earth-facing side. Only orbiting satellite and astronaut have seen the
back side of the Moon.
Because of the Moon rotates on its axis only once every 27 1/3 days, any
location on its surface experiences periods of daylight and darkness lasting
about two weeks. Along with the absence of atmosphere, accounts for the
high surface temperature of 1270C (2610F) on the day side of the moon
and the low surface temperature of -1730C (-2800F) on its night side.
Phases of the Moon The first astronomical phenomenon to be understood
was the regular cycle of the Phases of the Moon. On a monthly basis, we
observe the phases as a systematic change in the amount of the Moon that
Phases of the Moon We will choose the “new-Moon” position in the cycle
as the starting point.
About 2 days after the new Moon, a then silver (crescent phase) can be
seen with the naked eye low in the western sky just after sunset.
During the following week, the illuminated portion of the moon that is visible
from the Earth increases (waxing) to a half-circle (first-quarter phase) that
can be seen from about noon to midnight.
In another week, the complete disk (full-Moon phase) can be seen rising in
the east as the Sun sinks in the west.
During the next two weeks, the percentage of the Moon that can be seen
steadily declines (waning), until the Moon disappears altogether (new-
The lunar phases are a consequence of the motion of the Moon and the
sunlight that is reflected from its surface.
Half of the Moon that is illuminated at all times. But on the Earthbound
observer, the percentage of the bright side that is visible depends on the
location of the Moon with respect to the Sun and Earth.
When the moon lies between the Sun and Earth, none of its side faces
Earth, so we see the new-Moon (“no moon”) phase.
When the moon lies on the side of Earth opposite the Sun, all of its lighted
side faces Earth, so we see the full Moon.
Eclipses of the Sun and Moon When the Moon moves in a line directly
between Earth and the Sun, which can occur only during the new-Moon
phase, it casts a dark shadow on Earth, producing a solar eclipse
(eclipsis=failure to appear)
The Moon eclipse (lunar eclipse) when it moves within Earth’s shadow, a
situation that is possible only during the full-Moon phase.
The Moon’s orbit is inclined about 50 to the plane of the ecliptic. Thus, a)
during the most new- Moon phases, the shadow of the Moon passes either
above or below Earth. b) During most full- Moon phases, the shadow of
Earth misses the Moon.
An eclipse can only take place when a new- or full Moon phase occurs
while the Moon’s orbit crosses the place of the ecliptic.
Crossing the plane of ecliptic are normally met twice a year, therefore the
usual number of eclipses is four. These occur a set of one solar and one
lunar eclipse, followed six months later with another set. Occasionally the
alignment is such that three eclipses can occur in a one month period –at
the beginning, middle, and the end. These occur as a solar eclipse flanked
by two lunar eclipse, or vice versa.
It also occasionally happens that the first set of eclipses for the year occurs
at the very beginning of a year, the second set in the middle, and the third
set occurs before the calendar year ends, resulting in six eclipses in the
year. More rarely, if one of these sets consists of three eclipses, the total
number of eclipse is a year can reach seven, which is the maximum.
Total eclipses are visible only to people in the dark part of the Moon’s
shadow (umbra), while a partial eclipse is seen by those in the light portion
SAQ’s 1. Why do we use the mean solar day to measure time rather than
the sidereal day? 2. Why did the ancients believed that celestial objects
had some control over their lives? 3. What major change did Copernicus
make in the Ptolemaic system? Why was this change philosophically
4. What was Tycho Brahe’s contribution to science? 5. Does Earth move
faster in its orbit near perihelion (January) or near aphelion (July)? Keeping
your answer to previous question in mind, is the solar day longest in
January or July? 6. Use Kepler’s third law (p2 = d3 ) to determine the
period of a planet whose solar distance is: a) 10 AU b) 1 AU c) 0.2 AU
7. Use Kepler’s third law to determine the distance from the sun of a planet
whose period is a) 5 years b) 10 years c) 10 days 8. Did Galileo invent the
telescope? 9.Of what value are constellations to modern-day astronomers?
10. Explain the difference between the mean solar day and the sidereal
day. 11. What is the different about the crescent phase that precedes the
new-Moon phase and that which follows the new-Moon phase? 12. What
phases of the Moon occurs approximately one week after the new- Moon?
13. Currently, Earth is closest to the sun (perihelion) in January (147 million
km) and farthest from the sun in July (152 million km). As the result of the
precession of Earth’s axis, 12,000 years from now perihelion ( closest) will
occur in July and aphelion (farthest) will take place in January. Assuming
no other changes, how might this change average summer temperature for
your location? What about average winter temperature? What might the
impact on the biosphere and hydrosphere?
14. In what ways do the interactions between Earth and its Moon influence
the Earth system? If Earth did not have a Moon, how might the
atmosphere, hydrosphere, geosphere, and biosphere be different? 15.
Describe the locations of the Sun, Moon, and Earth during a solar eclipse
and during a lunar eclipse.