Encyclopedia of the Solar System 2nd ed

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CHAPTER 3

A History of Solar

System Studies

David Leverington

BAE Systems, England (retired)

1. Babylonians and Greeks 4. Second Half of the 17th Century 7. The 20th Century prior to the Space Age


2. Copernicus and Tycho 5. The 18th Century


3. Kepler and Galileo 6. The 19th Century

This chapter gives a brief overview of the history of solar
system research from the earliest times up to the start of
the space age.

1. Babylonians and Greeks

Many early civilizations studied the heavens, but it was the
Babylonians of the first millenniumb.c.who first used
mathematics to try to predict the positions of the Sun,
Moon, and visible planets (Mercury, Venus, Mars, Jupiter,
and Saturn) in the sky. In this they differed from the Greeks,
as the Babylonians were priests trying to predict the move-
ment of the heavenly bodies for religious purposes, whereas
the Greeks were philosophers trying to understand why they
moved in the way they did. The Babylonians were fascinated
by numbers, whereas the Greeks were more interested in
geometrical figures.
The accuracy of the Babylonian predictions in the 2nd
centuryb.c.is remarkable. For example, their estimate of
the length of thesiderealyear was within 6 minutes of its
true value, and that of the averageanomalistic monthwas
within 3 seconds. In addition, Jupiter’s sidereal andsynodic
periodswere within 0.01% of their correct values.
Pythagoras (c. 580–500b.c.) was a highly influential early
Greek philosopher who set up a school of philosophers,
now known as the Pythagoreans. None of Pythagoras’ orig-
inal writings survive, but later evidence suggests that the

Pythagoreans were probably the first to believe that the

Earth is spherical, and that the planets all move in separate

orbits inclined to the celestial equator. But the Pythagorean

spherical Earth did not spin and was surrounded by a se-

ries of concentric, crystalline spheres supporting the Sun,

Moon, and individual planets. Each had its own sphere,

which revolved around the Earth at different speeds, pro-

ducing a musical sound, the ”music of the spheres,” as they

went past each other.

Hicetus of Syracuse (fl. 5th centuryb.c.) was the first per-

son to specifically suggest that the Earth spun on its axis, at

the center of the universe. This model was further devel-

oped by Heracleides who proposed that Mercury and Venus

orbited the Sun as it orbited the Earth. Then Aristarchus

(c. 310–230b.c.), who was one of the last of the Pythagore-

ans, went one step further and proposed a heliocentric (i.e.,

Sun-centered) universe in which the planets orbit the Sun in

the (correct) order of Mercury, Venus, Earth, Mars, Jupiter

and Saturn, with the Moon orbiting a spinning Earth. This

was 1700 years before Copernicus came up with the same

idea. Aristarchus was also the first to produce a realistic esti-

mate for the Earth–Moon distance, although his estimate of

the Earth–Sun distance was an order of magnitude too low.

While the Pythagoreans were developing their ideas,

Plato (c. 427–347b.c.) was developing a completely dif-

ferent school of thought. Plato, who was a highly respected

philosopher, was not too successful with his geocentric (i.e.,

Earth-centered) model of the universe. His main legacy to

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