By the Babylonian priests the heavens were made the object of expert observation as early as 3800 B.C. The length of the year, the length of the month, the coming of the seasons, the course of the sun in the heavens, the movements of the planets, the recurrence of eclipses, comets, and meteors, were studied with particular care. One motive was the need of a measurement of time, the same motive as underlies the common interest in the calendar and almanac. It was found that the year contained more than 365 days, the month (synodic) more than 29 days, 12 hours, and 44 minutes. The sun's apparent diameter was contained 720 times in the ecliptic, that is, in the apparent path of the sun through the heavens. Like the Egyptians, the Babylonians took special note of the stars and star-groups that were to be seen at dawn at different times of the year. These constellations, lying in the imaginary belt encircling the heavens on either side of the ecliptic, bore names corresponding to those we have adopted for the signs of the zodiac,--Balance, Ram, Bull, Twins, Scorpion, Archer, etc. The Babylonian astronomers also observed that the successive vernal (or autumnal) equinoxes follow each other at intervals of a few seconds less than a year.
A second motive that influenced the Babylonian priests in studying the movements of the heavenly bodies was the hope of foretelling events. The planets, seen to shift their positions with reference to the other heavenly bodies, were called messengers, or angels. The appearance of Mars, perhaps on account of its reddish color, was associated in their imaginations with war. Comets, meteors, and eclipses were considered as omens portending pestilence, national disaster, or the fate of kings. The fortunes of individuals could be predicted from a knowledge of the aspect of the heavens at the hour of their birth. This interest in astrology, or divination by means of the stars, no doubt stimulated the priests to make careful observations and to preserve religiously the record of astronomical phenomena. It was even established that there is a cycle in which eclipses, solar and lunar, repeat themselves, a period (_saros_) somewhat more than eighteen years and eleven months. Moreover, from the Babylonians we derive some of our most sublime religious and scientific conceptions. They held that strict law governs the apparently erratic movements of the heavenly bodies. Their creation myth proclaims: "Merodach next arranged the stars in order, along with the sun and moon, and gave them laws which they were never to transgress."
