Astronomy 100 -- Early Astronomy

"EARLY ASTRONOMY"

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MESOPOTAMIAN ASTRONOMY

Some Examples of Astronomical Cycles They Discovered
• 47-Year Cycle for Mars.
  Crosses the meridian at midnight in the exact same position with respect to the stars (same constellation) every 47 years.

• SAROS
  Time interval between series of eclipses (18 years, 11-1/3 days).
  
  (For solar eclipses to occur at the same place on Earth, 3 Saros cycles must pass)

Implications of Identifying Cycles

1. Excellent calendars had been developed.
2. Accurate and readable records were kept.
   (47 year cycle implies accurate records for at least 100 years were kept of the position of Mars.)
3. Predictability of astronomical events was possible.

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ARGUMENTS AGAINST A MOVING EARTH

1. Earth was heavy!
   Based on Aristotle's "Fire, Water, Air and Soil".
   Soil was the heaviest, so why should it be moving?

2. Where is the wind?

3. Stars always have the same brightness and same positions. If the Earth was moving around the Sun, stars should get brighter and farther apart when the Earth moves closer to the stars. This was NOT observed. (This is the only "scientific" point in these three arguments).

Fix Figure 3-13

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ARISTOTLE [384 - 322 B.C.]

PROOFS OF A ROUND EARTH

1. Earth's shadow on the Moon during a Lunar Eclipse is ALWAYS round.
   Based largely on the observations of Anaxagorus.
   

2. Some stars seen in Egypt cannot be seen in Greece (Height of the Celestial Equator is different}.
If the Earth was flat, all the same stars should be visible.

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ERATOSTHENES [276 - 195 B.C.]

THE SIZE OF THE EARTH

Knew the Earth was round (from Aristotle's work)

Used the difference of the angles of the Sun's shadow at different places.
This is equal to the difference in LATITUDE!

Fix Figure 3-14

Eratosthenes determined the latitude difference between Alexandria and Syene to be 7.2°, or 1/50 of a circle (which is very accurate). But, he did not know the distance between Alexandria and Syene very well.

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ARISTARCHUS [310 - 230 B.C.]

SIZE OF AND DISTANCE TO THE MOON

Need to determine:
1. Angular size of the Moon (Easy!).
2. Linear size of the Moon (Not so easy!).

Used Lunar Eclipses to determine how many Moons fit across the shadow of the Earth. If the linear size of the Earth's shadow is known, from the size of the Earth, then the linear size of the Moon can be determined.

Fix Figure 3-09

Hipparchus used this method to determine the Earth-Moon distance to be 59 Earth radii (The value is actually 60.2).

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SIZE OF AND DISTANCE TO THE SUN

Need to measure the angle between the Sun and the Moon at exactly First or Third Quater (when the Sun-Moon-Earth angle is 90 degrees.)

Aristarchus measured an angle of 87 degrees implying the Sun was 50 times farther away than the Moon (the actual distance is more like 400 times farther).

Result was still important. If the Sun is 50 times farther away than the Moon, it must be 50 times larger than the Moon in order to have the same size on the sky. Since the Earth is only about 3 times larger than the Moon, the Sun must be about 17 times larger than the Earth. Based on this result, Aristarchus proposed a Sun-centered model of the Solar System.

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HIPPARCHUS [around 135 B.C.]

• First known catalog of stars. Contained positions and an "estimate" of brightness.
  (Basis of the Magnitude System used today!)

• Based on a comparison of his star positions and positions recorded 170 years earlier, he discovered the PRECESSION of the Earth.

  This PRECESSION is the slow circular motion of the North and South Celestial Poles with a period of 26,000 years! The North Star (Polaris) has NOT always been the Pole Star!

Fix Figure 3-15

Are we in the "Age of Aquarius?

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PTOLEMY [around 140 A.D.]

PTOLEMAIC (Geocentric)
MODEL FOR THE SOLAR SYSTEM

Geocentric system is an Earth-centered system.

Problems that needed to be solved for the Geocentric system:

PROBLEM 1. RETROGRADE MOTION OF MARS!

When the position of Mars is tracked over a period of several months, Mars sometimes appears to move "backwards" on the sky!!!

Mars in retrograde in early 2010.

These "retrograde loops" do not always look the same!
Fix Figure 2-28

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Ptolemy's Solution:
Planets move on EPICYCLES which move along DEFERENTS. The motion along the epicycle produces loops which were used to explain the retrograde motion.

Fix Figure 3-17

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PROBLEM 2. Mercury and Venus Always Near Sun

Ptolemy also had to explain why Mercury and Venus are only seen in the early morning or early evening sky and are NEVER in the sky in the middle of the night (they are never on the opposite side of the sky from the Sun).



Ptolemy's Rule: The centers of the epicycles for Mercury and Venus must stay on the line connecting Earth and the Sun.

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THE PTOLEMAIC SOLAR SYSTEM

Fix Figure 3-18

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Lunar and Solar Eclipses

The Astronomical Revolution

Back to The Front Page.

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