The history of
astronomy comprises three broadly defined areas that have characterized the science of the heavens since its
beginnings. With varying degrees of emphasis among particular civilizations and during particular historical periods astronomers have sought to understand the motions of
celestial bodies, to determine their physical characteristics, and to study the size and structure of the universe. The latter study is known as cosmology.
MOTIONS OF SUN, MOON, AND PLANETS
From the dawn of civilization until the time of Copernicus
astronomy was dominated by the study of the motions of celestial bodies. Such work was essential for astrology, for the determination of the calendar, and for the prediction of eclipses, and it was also fueled by the desire to reduce irregularity to order and to predict positions of celestial bodies with ever-increasing accuracy. The connection between the calendar and the motions of the celestial bodies is especially important, because it meant that astronomy was essential to determining the times for the most basic functions of early societies, including the planting and harvesting of crops and the celebration of religious feasts.
The celestial phenomena observed by the ancients were the same as those of today. The Sun progressed steadily westward in the course of a day, and the stars and the five visible planets did the same at night. The Sun could be observed at sunset to have moved eastward about one degree a day against the background of the stars, until in the course of a year it had completely traversed the 360¡ path of constellations that came to be known as the zodiac. The planets generally also moved eastward along the zodiac, within 8¡ of the Sun's apparent annual path (the ecliptic), but at times they made puzzling reversals in the sky before resuming their normal eastward motion. By comparison, the Moon moved across the ecliptic in about 27 1/3 days and went through several phases. The earliest civilizations did not realize that these phenomena were in part a product of the motion of the Earth itself; they merely wanted to predict the apparent motions of the celestial bodies.
Although the Egyptians must have been familiar with these general phenomena, their systematic study of celestial motions was limited to the connection of the flooding of the Nile with the first visible rising of the star Sirius. An early attempt to develop a calendar based on the Moon's phases was abandoned as too complex, and as a result astronomy played a lesser role in Egyptian civilization than it otherwise might have. Similarly, the Chinese did not systematically attempt to determine celestial motions. Surprising evidence of a more substantial interest in astronomy is found in the presence of ancient stone alignments and stone circles (see megalith) found throughout Europe and Great Britain, the most notable of which is Stonehenge in England. As early as 3000 ©, the collection of massive stones at Stonehenge functioned as an ancient observatory, where priests followed the annual motion of the Sun each morning along the horizon in order to determine the beginning of the seasons. By about 2500 ©, Stonehenge may have been used to predict eclipses of the Moon. Not until 1000 ¥ were similar activities undertaken by New World cultures (see archaeoastronomy).