The ancient Greeks

Of all the ancient civilizations, the Greeks probably made the most significant advances in astronomy, as they did in many other sciences, all of which they regarded as part of natural philosophy. Their civilization began to emerge as a major culture in about 900 B.C. and lasted until about 146 B.C., when the Romans took control. Greek astronomy began to develop as a science between about 600 B.C. and 450 B.C., although, in general, most of the Greeks’ important scientific achievements were made between 300 B.C. (after the conquests of Alexander the Great in the Middle East and India) and A.D. 200. During this period, the Greeks not only made numerous contributions of their own, but also collected and preserved knowledge from other cultures with which they came into contact.
The focal point of the Greek civilization was the city of Alexandria (the present-day chief port of Egypt). In its famous library was amassed (and later progressively destroyed) the knowledge of the Greeks, Phoenicians, Babylonians, Indians, as well as, later, that of the Arabs.
It was the mathematicians of India who developed the first really practical number system although in the third century B.C. Archimedes devised one that enabled large numbers to be manipulated relatively easily. (He also made one of the first calculations of the size of the universe.) The Greeks also developed algebra, which rapidly became invaluable to astronomers.
The geocentric universe
One of the earliest Greek scientists was Pythagoras, who lived in the sixth century B.C. He is now best known for his theorem concerning right-angled triangles. This extremely important theorem introduced the concept of numbers into geometry, thereby producing trigonometry, the basis of positional astronomy and of the modern star coordinate system.
Pythagoras also put forward the erroneous idea that the earth is the unmoving center of the universe—a proposal known as the geocentric theory of the universe. This idea received widespread support from Aristotle, Ptolemy, and Eudoxus of Cnidus (who in the fourth century B.C. proposed a complex model of the universe consisting of 27 interconnected concentric spheres with the earth at their common center), among other influential thinkers. This remained the generally accepted theory of the universe until it was finally disproved by Nicolaus Copernicus (1473-1543).

Nevertheless, some Greek scientists did question the geocentric theory, notably Aristarchus of Samos who, in about 280 B.C., proposed what we now know to be the truth—that the earth revolves around the sun. Such dissenting opinions were ridiculed, however, because it could be plainly seen that the earth was stationary.

Aristarchus of Samos not only proposed a heliocentric model of the solar system (A, top), but also tried to determine the relative distances to the moon and sun (A, bottom). Assuming the angle the moon at first quarter makes with the earth and sun to be 90°, he measured the moon-earth-sun angle to be 87°. From this he calculated the ratio of the distances to the sun and moon from earth to be 19:1. In fact, the true angle to the sun from earth is 89° 51′, and the correct ratio of distances is 390:1. Eratosthenes, the librarian of Alexandria, made the first accurate determination of the earth’s circumference (B). Noticing that the midday sun’s reflection was visible in the water of a deep well at Syene, he correctly reasoned that the sun’s beams must point directly toward the earth’s center. Hethen measured the angle to the sun at Alexandria—480 miles (770 kilometers) away—to be 7.2° (^j of a circle) and from this he calculated the earth’s circumference to be 480 miles X 50 = 24,000 miles (38,620 kilometers). The correct figure is about 24,800 miles (39,900 kilometers). In Ptolemy’s version of the erroneous geocentric theory (C, top), Earth was orbited by the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn. The orbits were epicyclic (C, bottom); each planet circled a point that itself revolved around the earth.

Greek observations

The ancient Greeks made more positive progress in practical astronomy. Eratosthenes, the librarian of Alexandria, pioneered the use of geometry in making calculations from astronomical observations. For instance, he calculated the earth’s circumference to be about 24,000 miles (38,600 kilometers)—close to the correct figure, 24,901.55 miles (40,075.16 kilometers), at the equator.
Hipparchus made an important contribution when in about 130 B.C. he used stereographic projection to make a map of the stars. This is a good method for translating the positions of stars, assumed to lie on a sphere, onto a plane surface because it results in relatively little distortion. Also, lines of azimuths are arcs of circles and hence easy to draw accurately. The method continued to be used to make star maps until the seventeenth century—for example, on the metal star maps of astrolabes. Hipparchus also compiled a star catalogue (later augmented by Ptolemy). From his many naked-eye observations he discovered that the celestial pole moves by a few minutes of arc over a period of several years, a phenomenon now called precession of the equinoxes. He also noted the corresponding slight annual recession of the vernal and autumnal equinoxes (the times of year when day and night are of equal length throughout the world).
Ptolemy was one of the last great ancient Greek astronomers. He made an enormous number of observations and recorded the positions of the stars in his famous Almagest, which he compiled in about A.D. 150. In this book, he also elaborated the geocentric theory of the universe and attempted to account for the motions of celestial objects. Like other Greek astronomers before him, he rejected the idea that the earth moves in space. He explained the apparently irregular motions of the planets by ascribing to them epicyclic orbits, in which each planet moves in a small circle around a point that itself moves around a larger circle. (The concept of epicyclic motion was originally developed in about 230 B.C. by Apollonius of Perga.) In this way, Ptolemy maintained the philosophical ideal of perfection, which dominated ancient Greek, and medieval, thought.

The astrolabez a type of star map, was developed by the ancient Greeks as a combination of three more primitive instruments: the armillary sphere, a skeleton celestial globe; the equinoctial armillary, a metal ring used to determine the arrival of the equinoxes; and the solstitial armillary, a series of scaled rings used to measure the altitude of the sun. Medieval astrolabes (one made in 1548 is shown at left) bore a stereographic projection of the celestial sphere. A typical astrolabe consists of a horizon circle graduated in degrees (or azimuths) from the north point, and star altitude circles. There is also a sighting bar to measure the altitudes of stars and other