Greek Astronomy

Modern astronomy can trace its heritage directly back to the ancient Greeks, who began to develop explanations for their observations of the sky. The writings of Aristotle summarize the knowledge of that era. He attributed the phases of the Moon—that is, the changes in its apparent shape—to the fact that we see different portions of its sunlit surface during the month. He also knew that the Sun is farther away from the Earth than the Moon because the Moon occasionally passes between the Sun and Earth and blocks the Sun’s light (a solar eclipse). 

Aristotle cited two observations to show that Earth is a sphere. The first is that the shadow of Earth, which is seen during an eclipse of the Moon (when Earth is directly between the Sun and Moon), is always round. Only a sphere always has a round shadow no matter how it is viewed. If the Earth were a disk, we would sometimes see the shadow edge-on, and it would look like a straight line. The second observation was that travelers who journeyed a long distance south reported seeing stars not visible from Greece. If Earth were flat, all travelers anywhere would see the same stars. On a spherical Earth, travelers at different latitudes (different distances north or south) view the sky from different angles and see different constellations.

The Greek astronomer and mathematician Eratosthenes measured the size of the spherical Earth in about 200 bc. He noticed that on the first day of summer in Syene, Egypt, the Sun was directly overhead at noon. On the same date and time in Alexandria, Egypt, the Sun was about 7 degrees south of zenith. With simple geometry and knowledge of the distance between the two cities, he estimated the circumference of the Earth to be 250,000 stadia. (The stadium was a unit of length, derived from the length of the racetrack in an ancient Greek stadium. We have an approximate idea of how big an ancient Greek stadium was, and based on that approximation Eratosthenes was within 20 percent, and possibly within 1 percent, of the correct answer.) 

Probably the most original ancient observer of the heavens was Aristarchus of Sámos, a Greek. He believed that motions in the sky could be explained by the hypothesis that Earth turns around on its axis once every 24 hours and, along with the other planets, revolves around the Sun. This theory, however, makes an important prediction that ancient Greeks could not verify. If Earth moves in an orbit around the Sun, then we look at the stars from different directions at different times of the year. As Earth moves along, nearby stars should shift their positions in the sky relative to more distant ones. The Greeks tried to measure this effect for the stars but were unsuccessful. It was only in 1838 that astronomers’ equipment could make measurements with the accuracy required to measure the very small shift of the stars, which turn out to be much, much farther away than the Greeks could imagine.

Perhaps the greatest of the ancient astronomers was Hipparchus, who lived around 150 bc and did most of his work at an observatory he built in Rhodes. There he recorded accurate positions of about 850 bright stars and classified them according to their brightness. The brightest stars he said were of the first magnitude, a term astronomers still use today. Because our planet is not an exact sphere, but bulges at the equator, the gravitational pulls of the Sun and Moon cause it to wobble like a top. It takes about 26,000 years for Earth’s axis to complete one full circle. Hipparchus estimated that the Earth’s axis shifts its position relative to the stars by 46 seconds of arc per year, which is very close to the modern value of 50.26 seconds of arc per year. This is known as the precession of the Earth.

The last of the great ancient astronomers was Ptolemy, who worked in Alexandria in about the year ad 140. Ptolemy’s greatest contribution was a geometrical model of the solar system that made it possible to predict the positions of the planets at any date and time. His model was used for about 1,400 years, until the time of Copernicus. Ptolemy’s challenge was to explain the complex motions of the planets, including the fact that they sometimes appear to move westward or backward in their orbits. In order to explain the observation, he assumed that each planet revolved in a small orbit called an epicycle. The center of the epicycle then revolved about the Earth on a much larger circle. At the time, circles were thought to be the perfect shape. It was assumed that the heavenly bodies would follow the most perfect shape. See also Ptolemaic System.

Astronomers now know that the planets do not follow circular orbits but rather elliptical ones, and they orbit around the Sun, not Earth. The backward or westward motion is explained by the fact that Earth moves more rapidly in its orbit than do Mars, Jupiter, and Saturn. When the Earth overtakes them during its yearly circuit around the Sun, these planets appear to move backwards relative to the stars. For an analogy, think of passing a slowly moving car on the freeway. As you overtake it, the car appears to be moving backward relative to the scenery beyond the side of the road.