Archaeologists think Stonehenge had astronomical significance. It was certainly built with astronomical phenomena in mind. One pillar, called the Heel Stone, appears to be near the spot where sunlight first strikes on the summer solstice. Thus, Stonehenge may have served as a sort of calendar. Other evidence suggests that Stonehenge may have been used as a predictor of lunar eclipses.

About 140 C.E. the ancient Greek astronomer Claudius Ptolemy, who lived and worked in Alexandria, Egypt, published a thirteen-volume treatise on mathematics and astronomy called Megale mathmatike systaxis (“The Great Mathematical Compilation”), which is better known today as The Almagest. In this work, Ptolemy built upon—and in some cases, probably reprised—the work of many predecessors, such as Euclid, Aristotle, and Hipparchus. He described a model of the cosmos, including the solar system, that became the astronomical dogma in Western civilization for more than one thousand years.

According to the Ptolemaic model, Earth stands at the center of the universe, and is orbited by the Moon, the Sun, Mercury, Venus, Mars, Jupiter, and Saturn. The stars in the sky are all positioned on a celestial sphere surrounding these other objects at a fixed distance from Earth. The planets follow circular orbits, with extra “additions” on their orbital paths known as epicycles, which explain their occasional retrograde motion through the sky. Ptolemy also cataloged more than one thousand stars in the night sky. Although the Ptolemaic model of the solar system was proven wrong by Galileo, Kepler, Newton, and other great scientists starting in the seventeenth century, it was very important for the development of astronomy as a modern science.