Astronomy 161:
An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 9:30 |

Greek Astronomy

- Anaximander
- Pythagoras, Eudoxus, & Aristotle

Early Heliocentric System:

- Aristarchus of Samos

Epicyclic Geocentric Systems:

- Hipparchus of Nicaea
- Claudius Ptolemy

- Uniform daily motion about the celestial poles.

**The Sun**:

- Daily motion around the celestial poles (rising and setting).
- Eastward drift along the Ecliptic over a year, a little faster in winter, slower in summer.

**The Moon**:

- Daily motion around the celestial poles.
- Eastward motion near the Ecliptic over a month.

**The Planets**:

- Daily motions about the celestial poles.
- Generally eastward motion near the Ecliptic at different speeds for each planet.
- Occasional westward "retrograde" motions.
- Superior Planets are brighter at opposition, fainter at conjunction.

Any successful description of the Solar System must explain all these facts.

Anaximander of Miletus (611-546 BC)

Among the first Greek philosophers to suggest a geocentric system:

- Earth was a flat disk (cylinder) fixed and unmoving at the center.
- Sun, Moon & Stars were affixed to rotating crystalline spheres centered on the Earth.
- Sun, Moon & Stars were
*physical objects*.

Pythagoras of Samos (569-475 BC)

Philosopher & Mathematician, founded the Pythagorean school. Taught that spheres are the perfect geometric shapes.

Pythagorean Model:

- Spherical Earth fixed at the center
- Planets & Stars on concentric crystalline spheres
- Sizes were ratios of small numbers (e.g., 2:1, 3:2)

Vibrations from their rubbing together created a harmonious "Music of the Spheres.

System of 27 Spheres:

- 1 for the fixed stars
- 3 each for the Sun and Moon
- 4 each for the 5 planets

Spheres within spheres in perfect circular motion combine to give retrograde motions.

Spheres within Spheres

(Click on the image to view at full scale [Size: 20Kb])

Four Spheres for each planet:

- One was aligned with the celestial poles, turning once a day to give rising & setting.
- Second was tilted 23.5°, rotated slowly in the opposite direction to give the usual west-to-east drift of the planets relative to the fixed stars.
- Third & Fourth were introduced to produce the periodic retrograde motions of the planets.

All were in uniform circular motion about their axes.

His *On the Heavens* refined Eudoxus' system:

- 55 crystalline spheres within spheres

Incorporated physical reasoning:

- Earth fixed and unmoving at the center as it was too big to move, including rotation.
- All spheres were in
*uniform circular motion*. - Combination of perfect motions produced net retrograde and non-uniform motions.

Earth, Air, Fire, & Water

The Aristotelian World View made certain basic assumptions:

- The Earth was a sphere, fixed and unmoving at the center of the Universe.
- The natural state of motion on the Earth was rest.
- The natural state of the Heavens was unceasing uniform circular motion (or combinations of such motions).

Preserving Appearances

The models proposed sought primarily to "preserve appearances":

- Started from philosophical ideals.
- Built systems to conform to these ideals.
- Adjusted to provide accurate predictions of planetary motions.
- No need to explain the physical "causes" of the motions.

Alternative viewpoint to Geocentric:

- Puts the Sun, not the Earth, at the center.
- The Earth rotates and revolves around the Sun.
- Stars are on an outermost celestial sphere.

The complex non-uniform and retrograde motions are now just a consequence of viewing moving planets from a moving Earth.

- Showed geometrically that the Sun was at least 20x further than the Moon.
- Really 400x further: sound method, but inadequate data.
- Meant Sun was 5x bigger than the Earth (more like 109x, again, sound method but inadequate data).
- Makes it even more absurd that the Sun should move, if the Earth was too large to move according to Aristotle.

(Click on the image to view at full scale [Size: 9Kb])

We know none of the details of Aristarchus' Heliocentric model. We only have his treatise on the distances and sizes of the Moon and the Sun, from which we gather what his motivations might have been. We know of his Heliocentric model only from mention of it (usually dismissively) by others who came after him. In particular, Archimedes discussion of it in his Sand Reckoner is our best source.

The heliocentric idea never caught on, perhaps because it was considered too radical given deeply ingrained notions about uniform circular motion and the immobility of the Earth.

- Discovered the Precession of the Equinoxes
- Developed the system of stellar magnitudes

Developed a New and Improved Geocentric System:

- Introduced epicycles, building on ideas of Apollonius of Perga.
- Located the Earth slightly off-center (eccentric)

(Click on the image to view at full scale [Size: 7Kb])

The main circle is the *Deferent*, upon which rotates the center of
the secondary circle called the *Epicycle* to which the planet is
affixed. The two rotate at different rates, fine-tuned to make the
apparent motions as seen from the Earth come out right. Additional
epicycles are added to further fine-tune the system. Notice that the
Earth is not exactly at the center, but slightly offset at the
"eccentric" point.

- Combined motion of deferent and epicycle produces retrograde motion.
- Superior planets are brighter at opposition, when moving retrograde.
- Placing the Earth at an eccentric away from the deferent center helps explain the observed non-uniform motions of the Sun, Moon, and Planets.
- Can fine-tune the model by adding epicycles

Note:

What distinguishes Hipparchus' geocentric model from all previous models is that it was firmly grounded in observational data, many observations of which he made himself (by all accounts Hipparchus was the supreme observational astronomer of the classical period). In many ways, this work marks a turning point between models motivated primarily by philosophical aesthetics and models based at least in part upon observational data. Hipparchus still sought to preserve appearances, and chose a reasonable (to him) geometric model to represent planetary motions mathematically.

Wrote the *Mathematical Syntaxis:*

- Compilation of all Mathematical & Astronomical knowledge of his time.
- Known to us in Arabic translations that hailed it "Al Magest" ("The Greatest").
- Vastly influential work in medieval Europe after the 11th century.

Elaborated Hipparchus' geocentric system, adding extra features to better preserve appearances.

The Equant

Ptolemy introduced the Equant, a geometric "device" to account for observed changes in a planet's speed as it moved around the Earth.

(Click on the image to view at full scale [Size: 6Kb])

- The Epicycle still moves about the center of the Deferent, but
- Uniform circular motion about the center of the deferent is replaced by uniform "angular" motion about an off-center equant point.

- 40 epicycles and deferents.
- Equants and eccentrics for all planets, the Moon, and Sun
- With only minor modifications, it provided accurate predictions of the motions of the planets, Sun, and Moon.

It was to prevail virtually unchallenged for nearly 1500 years.

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Updated: 2005 October 7

Copyright © Richard W. Pogge, All Rights Reserved.