astronomy

1. Chapter 1: History of Astronomy
Dr Remudin Reshid Mekuria
February 17, 2021

2. Questions to Explore
I What was different about the Greek view of the heavens
compared with that of their predecessors?
I How did the Greeks determine the size and shape of the Earth
nearly 2000 years before Columbus?
I Why did the Greeks reject the idea that the Earth moves
around the Sun?
I What was Ptolemy’s model of planetary motion and why was
it the standard for 1500 years?
I Why did Tycho Brahe, the great observational astronomer,
reject the heliocentric model of the solar system?
I How were Tycho’s observations used by Kepler to produce his
laws of planetary motion?
I How did the telescopic observations of Galileo support the
heliocentric model of the solar system?
I What finally convinced the world that the Earth revolves
around the Sun rather than the other way around?
Dr Remudin Reshid Mekuria Chapter 1: History of Astronomy

3. 1.1 The size and shape of the Earth: Ancient Astronomy
I The first astronomers to make long-term written records
(mostly lost or destroyed) of their observations were the
Mesopotamians, who began to build observatories about 6000
years ago.
I They introduced the 12 zodiacal constellations.
I They originated the idea of dividing a circle into 360° and then
further subdividing each degree into 60 parts called minutes
and each minute into 60 parts called seconds.
I A full circle, 360°, symbolized the annual motion of the Sun,
which takes about 360 days.
I With the proper mix of 12-month and 13-month years, it was
possible to make the average year have the correct length even
though individual years were either too short or too long.
I By about 500 B.C the Babylonians (one of the many cultures
in the long history of Mesopotamian civilization) repeatedly
determined the dates of the various configurations of the Sun,
the Earth, and the planets.
Dr Remudin Reshid Mekuria Chapter 1: History of Astronomy

4. I One of the main reasons that the Babylonians were so
interested in the positions of the Sun, Moon, and planets was
their belief in astrology.
I Astrology is a pseudoscience involving the belief that the
positions of the celestial objects influence events on the Earth.
I No scientific basis for astrology exists, and no controlled study
has ever shown the slightest indication that the predictions of
astrologers are any better than clever guesswork guided by
knowledge of human nature.
I The Babylonians’ were able to express the position of a planet
as the sum of a series of regular cycles, which took different
lengths of time to complete.
I They produced almanacs of the planets for key dates, such as
the beginning of each month: this predictions of the celestial
events marked the beginning of astronomy in the scientific
sense.
I Using similar technique for the motion of the Moon as that of
the planets, they were able to predict some lunar eclipses by
about 2700 years ago.
Dr Remudin Reshid Mekuria Chapter 1: History of Astronomy

5. I While the Mesopotamians were building their vast record of
astronomical observations, astronomers were also at work in
Egypt.
I The Egyptians developed and used astronomy entirely for
practical purposes, such as developing a calendar to be used
for predicting the Nile flood and in aligning temples and
monuments.
I The Egyptians must have used astronomical methods to align
their buildings, but no records have survived to tell us how
they did it and their reasons for doing so.
I We have a few manuscripts that tell us about their
mathematics, but almost no descriptions of their science and
engineering has survived.
I It is possible that much of what we usually consider Greek
astronomy was based on much earlier work done in Egypt.
Dr Remudin Reshid Mekuria Chapter 1: History of Astronomy

6. I The Greeks are generally acknowledged as the first people to
raise astronomy from the level of prediction to that of
explanation and understanding.
I The first of the Milesian (from city of Miletus in Greek)
astronomers was Thales (C.624 - 547 B.C.), a merchant who
was said to have traveled to Egypt to gather ancient
knowledge
I The most famous story about Thales is that he predicted a
total solar eclipse. The eclipse took place during a battle
between the Lydians and the Persians in 585 b.c. The two
armies were so awestruck by the eclipse that they put down
their arms and ended the battle.
I Mechanical explanations of astronomical phenomena were first
used by the astronomers of Miletus. Their ideas about the
stars eventually developed into the concept of the celestial
sphere.
I The idea that the celestial bodies were spheres and that they
moved on perfectly circular paths originated with Pythagoras
and his students.
Dr Remudin Reshid Mekuria Chapter 1: History of Astronomy

7. I Eudoxus devised a model in which the motion of a planet was
produced by the rotation of several spheres.
I Aristotle argued that all the celestial bodies were spheres and
moved on circular paths. He believed that the Earth was
motionless in the center of the universe.
I Aristotle argued that falling bodies move toward the center of
the Earth. Only if the Earth is a sphere can that motion
always be straight downward, perpendicular to the Earth’s
surface.
I The Earth’s shadow on the Moon during lunar eclipses is
always round. If the Earth were a flat disk, there would be
some eclipses in which the Earth would cast a flat shadow on
the Moon.
I Aristarchus showed that it is possible to use geometry to find
the distance of the Moon and the relative distances of the
Moon and Sun.
I By doing so he showed that the universe is enormous
compared with the size of the Earth.
Dr Remudin Reshid Mekuria Chapter 1: History of Astronomy

8. I Taking in to account the actual shape of the converging
shadow of the Earth. Once the size and angular diameter of
the Moon are known, its distance can be found as well.
I The distance to the Moon, the linear diameter of the Moon,
and the Moon’s angular diameter comprise two sides and an
angle of a long, skinny triangle, like that shown in the Figure
below.
I The small angle equation relates the distance to an object (d),
its linear diameter (D), and its angular diameter (θ)
θ = 206, 265 ×
D
d
, (1)
when θ is measured in seconds of arc. The number 206,265 is
the number of seconds of arc in one radian.
Chapter 1: The Earth

9. I Example 1: Venus has a linear diameter of 12,100. What is its
angular diameter when it is at a distance of 5 × 107 km?
I Solution:
θ = 206, 265 ×
12, 100km
5 × 107 km
= 50 seconds of arc
I Example 2: We can calculate the distance to a body if its
linear diameter and angular diameter are known. In the case
of the Moon, the linear diameter, D, is 3480 km. The angular
diameter of the Moon is 0.5°. Re-arranging equation 1 we
obtain:
d = 206, 265 ×
D
θ
, (2)
note In order to use this equation, the angular diameter of the
Moon, must be converted from degrees to seconds of arc
(0.5° = 1800 seconds of arc).
I The distance of the Moon is then given by
d = 206, 265 ×
D
θ
= 206, 265 ×
3480km
1800
= 400, 000km.
Chapter 1: The Earth

10. I Aristarchus proposed that the Sun, not the Earth, is the
center of the universe, but this idea was not accepted by most
other Greek astronomers.
I Eratosthenes found the difference in the altitude of the
noonday Sun at Syene and Alexandria. He realized that this is
the same as the difference in latitude of the two cities. This
allowed him to find the ratio of the circumference of the Earth
to the distance between Syene and Alexandria.
Chapter 1: The Earth

11. I The celestial coordinates of stars change with time because of
precession, the slow circular shifting of the celestial poles with
respect to the stars. Hipparchus discovered precession when
he compared his measurements of stellar positions with those
of earlier Greek astronomers.
I In Ptolemy’s model the retrograde motion of a planet was
produced by the combination of two circular motions. A
planet moved in a circle on an epicycle, which itself moved on
a deferent.
Chapter 1: The Earth

12. 1.2: Our place in the Universe
1.2.1: The Geocentric Model
I Greek astronomy culminated with the geocentric system of
Ptolemy.
I The Ptolemy’s model shown above could predict accurately
the positions of theplanets and was in use for nearly 1500
years.
I Although the system of Eudoxus could produce motions
resembling those of the planets, it really couldn’t stand a close
comparison to those observed motions.
I Ptolemy’s system, in contrast, was able to match closely the
motion of each planet and predict its future position.
I Ptolemy didn’t invent the system named after him, he only
completed it. Other astronomers, including Hipparchus, had
been developing the idea for centuries. The system is
geocentric, which means the Earth is stationary at the center.
Chapter 1: The Earth

13. I To justify the assumption that the Earth is stationary,
Ptolemy used essentially the same arguments that had been
used by Aristotle, namely that the Earth is too heavy to move
and there would be drastic effects if it rotated on its axis or
revolved around the Sun.
I The Complete Geocentric Model: Venus and Mercury always
move so that the centers of their epicycles lie on the
Earth-Sun line.
Chapter 1: The Earth

14. I After the time of Claudius Ptolemy, the astronomy of the
Greek and Roman world, like nearly every other aspect of
western civilization, went into a long decline.
I In the first stage of the decline, scientific activity became less
frequent and less innovative. Little new information was
discovered.
I In the second stage of the decline, astronomical knowledge
not only ceased to grow, but actually diminished.
I By the seventh century, Ptolemy was completely unknown in
the West and Aristotle was known only through a few small
works on logic.
I Part of the reason for the loss of Greek astronomical
knowledge can be attributed to the antagonism of the early
Christian church to many of the features of Greek astronomy.
Chapter 1: The Earth

15. I Although the astronomical works of the Islamic world from
the seventh to the fifteenth centuries were usually written in
Arabic, many of the Islamic astronomers weren’t Arabs.
I They worked in many different cities from Spain to central
Asia. The Islamic astronomers improved many observing
techniques and made careful observations.
I The discoveries of the ancient astronomers were preserved by
Islamic astronomers, who found them and translated them
into Arabic.
I The Islamic astronomers collected, translated, and
commented on the works of the ancient astronomers,
supplemented the ancient works with new observations, and
then passed their astronomy on to Christian Europe.
I In doing so, Islam exerted a powerful influence on the rebirth
of western astronomy.
Chapter 1: The Earth

16. I Astronomical knowledge was gradually reacquired in Western
Europe.
I By the fifteenth century, the astronomy of the ancients had
been rediscovered by Europeans.
I Astronomers had begun to observe again and to test
hypotheses against observations.
I The geocentric model of Ptolemy was accepted by nearly all
astronomers.
I Some astronomers, however, had growing doubts about
Aristotle’s theory of motion and his arguments that the Earth
must be motionless.
I For e.g. Aristotle taught that unless an object was moved by
an external push or pull it could only remain at rest or fall
directly to Earth.
I Why, then, doesn’t an arrow fall directly to the ground as
soon as it is released by a bow?
Chapter 1: The Earth

17. I Nicholas Copernicus (1473 - 1543), the astronomer who
proposed the first fully developed heliocentric model of the
solar system, was born in Torun, in what is now Poland.
I He proposed that the Sun rather than the Earth is the center
of the solar system.
I In the heliocentric model, the daily and annual patterns of
celestial motion are explained by the rotation and revolution
of the Earth.
I Retrograde motion of the planets occurs whenever the Earth
passes or is passed by another planet.
I This can be explained using this model quite easily! To be
demonstrated in class.
I In the model of Copernicus, the orbital distances of the
planets can be found through observations and geometry.
I In contrast, the geocentric model makes no specific
predictions about the relative distances of the planets.
Chapter 1: The Earth

18. I The Parallax of a Star: A consequence of the Earth’s motion
about the Sun is that a nearby star is seen in slightly different
directions when the Earth is on opposite sides of the Sun.
Chapter 1: The Earth

19. I Through his care in building and using astronomical
instruments, Tycho Brahe was able to make observa- tions of
unparalleled accuracy.
I His regular observations of the Sun, Moon, and planets
covered many years.
I His data replaced the ancient observations that earlier
theorists had been using for centuries.
I Tycho was unable to detect stellar parallax and thus rejected
the model of Copernicus. –Once again we moved back in
history!!
I Tycho proposed a model in which the Earth is orbited by the
Sun and Moon but all of the other planets move about the
Sun.
I So this model is compromised between the geocentric model
of Ptolemy and the heliocentric model of Copernicus.
Chapter 1: The Earth

20. I Using Tycho’s data, Kepler was able to discover the laws of
planetary motion.
I Kepler’s first law says that the planets move on elliptical paths
with the Sun at one focus.
I Kepler’s second law says that a planet moves so that a line
drawn between the planet and the Sun sweeps out equal areas
in equal amounts of time.
I This means that the product of transverse velocity and
distance from the Sun remains constant as a planet moves
about the Sun.
I The planet moves fast- est when it is nearest the Sun.
I Kepler’s third law says that the square of the sidereal period
of a planet is proportional to the cube of it average distance
from the Sun.
I The third law implies that there is a common principle that
governs the orbital motions of the planets.
Chapter 1: The Earth

21. I It is likely that Kepler’s discoveries alone eventually would
have resulted in the abandonment of the geocentric model of
the solar system.
I The triumph of the heliocentric model, however, was also
aided by a series of observations made by Galileo Galilei
(1564-1642) shortly after the telescope was invented.
I In particular, his observations that Venus shows all the phases
from new to full could not be explained by PtolemyâĂŹs
model of the solar system.
I Galileo summarized his arguments for the heliocentric model in
his book Dialogue Concerning the Two Chief World Systems.
I The book put Galileo in conflict with church authorities and
resulted in his persecution.
Chapter 1: The Earth

22. 1.3 : Motion of the Earth
I Under this sub-topic, we will try to address the following
problems, namely:
I How do we know that the planet Earth really revolves and
rotates?
I What is the structure of the interior of the Earth?
I What causes mountain ranges, volcanos, and earthquakes?
I In the Copernican model of the solar system, the Earth
revolves around the Sun and rotates about its polar axis.
I For almost 200 years after the death of Copernicus, there was
no real evidence that the Earth was a revolving, rotating
planet.
I Today, however, we have compelling evidence of the Earth’s
revolution and rotation.
Chapter 1: The Earth

23. 1.3.1 : The rotation of the Earth
I A Foucault Pendulum: As the pendulum swings back and
forth, the Earth rotates underneath it so that the direction of
the pendulum’s swing appears to change.
I Its direction of swing completes a full circle in one sidereal
day, 23 hours 56 minutes.
I The Earth’s rotation distorts the shape of the planet from a
sphere to an oblate spheroid:
I The inertia of the Earth itself causes it to be deformed from a
spherical shape.
I The consequences of inertia are greatest at the equator, where
the speed at which the Earth’s surface rotates is largest.
I The effect falls to zero at the poles. Because of its rotation,
the Earth has an oblate shape.
Chapter 1: The Earth

24. I The Coriolis effect is another fictitious force caused by the
Earth’s rotation.
I The Coriolis effect appears to curve the paths of moving
objects, winds, and water currents to the right in the northern
hemisphere and to the left in the southern hemisphere.
I The Coriolis effect causes moving objects and air masses to
appear to be deflected from straight line paths.
I It is responsible for spiral-like circulation patterns in the
atmosphere and oceans.
I See Fig. 8.6 to 8.8 of your text book.
Chapter 1: The Earth

25. 1.3 : 2 The revolution of the Earth and the seasons
I For several centuries after Copernicus died, astronomers
(including Tycho Brahe) tried to verify that the Earth orbited
the Sun by searching for stellar parallax.
I Ironically, the first evidence of the Earth’s revolution resulted
from an unsuccessful search for stellar parallax in the 1720s by
the English astronomers Samuel Molyneux and James Bradley:
I As the Earth moves in its orbit, the positions of stars appear
to shift back and forth just as the apparent direction of falling
raindrops changes as we move back and forth during a
rainstorm.
I Bradley reasoned that the shifting positions of the stars were
due to the changing direction in which the Earth moved
around the Sun.
I This effect, the aberration of starlight, was the first direct
evidence for the revolution of the Earth.
I See Fig. 8.1 to 8.2 of your text book.
Chapter 1: The Earth

26. The Celestial Sphere
Chapter 1: The Earth

27. I The Sun appears to move eastward along the ecliptic relative
to the stars.
I The ecliptic is inclined 23.5° with respect to the celestial
equator
I The solstices occur when the Sun is farthest north or south of
the celestial equator.
I The equinoxes occur when the Sun crosses the celestial
equator.
I The changing declination of the Sun affects the point on the
horizon where the Sun rises (the azimuth of sunrise) and the
duration of daylight.
I When it is north of the celestial equator, the Sun rises in the
northeast and daylight lasts for more than half the day for
people in the northern hemisphere.
I When south of the equator, the Sun rises in the southeast and
day is shorter than night.
Chapter 1: The Earth