Stars over Ancient Babylon

Intervening dynasties, 1600 - 700 BC

For centuries after the Old Babylonian period, amid the tumult of revolutions of empires, through a variety of lesser known dynasties, the scribes sustained their careful inquiry into mathematics and astronomy.

Because the scribes provided counsel for the king, their inquiry was of great importance to the empire. The scribes advised the king by divination, which is the art of interpreting omens such as dreams. The scribes also practiced divination by the stars, or astrology. To interpret the meaning of the stars, astrologers required a knowledge of astronomy as a mathematical science. Ancient astronomy and astrology were thoroughly mixed together. Modern astronomy and astrology have no relation; there is no physical reason to expect the positions of the stars and planets to cause events on the Earth. Yet this modern separation between astronomy and astrology does not mean that astronomy began only when astrology was set aside; rather, astrology remained the most important incentive for the development of mathematical astronomy up through early modern times.

In the Kassite dynasty, from 1600 to 1200 BC, the scribes of Enuma Anu Enlil compiled 70 tablets containing the interpretations of thousands of omens. These tablets were an experiment to collect observations of the Moon and planets, and then to correlate these various omens with economical prosperity, agricultural prices, civic health, and affairs of state, all of which were assiduously recorded.

Explore a tablet of the Enuma Anu Enlil series at the British Museum.

The tablets of Enuma Anu Enlil record observations of a wide variety of astronomical phenomena that the scribes believed might serve as possible omens. For example, the scribes observed the heliacal risings of bright stars such as Regulus, in Leo the Lion. As the Sun moves around the sky once each year, it will sometimes be found on the opposite side of the sky from a star such as Regulus. When the Sun and Regulus are thus in “opposition,” so that the angle between them is 180°, Regulus will rise when the Sun sets, and Regulus will be visible all night long.

Each night the Sun moves about one degree in a roughly eastward direction along its annual path known as the “ecliptic.” In 10 days, the Sun will move about 10 degrees, roughly the width of one’s fist held at arm’s length.

Six months later, as the Sun approaches Regulus, Regulus and the other stars of Leo will disappear into the daytime sky. Eventually, as the Sun continues to travel roughly eastward about a degree a day, the Sun will pass them by. A morning will come when Regulus rises on the eastern horizon, just before sunrise. This first appearance of Regulus after its period of invisibility in the daytime sky is called its “heliacal rising.” After its heliacal rising, Regulus is a morning star, visible in the east before sunrise. The Sun returns to the same place against the background of fixed stars each year, and the heliacal risings of important bright stars occur at certain fixed times of the year.

Heliacal Rising demo (requires QuickTime)

The scribes of Enuma Anu Enlil also watched the skies to observe the motions of the Moon and planets. We have seen that the Sun moves roughly eastward about a degree a day along the ecliptic, its annual path around the sky. Similarly, each planet and the Moon move roughly eastward, near the ecliptic, with what is called their “direct” motion. For example, on average the Moon moves about 10 degrees a day, roughly the width of one’s fist held at arms length. If tonight the Moon is located a few degrees west of a star, then tomorrow night it will be a few degrees east of the same star. At this pace the Moon completes its journey around the sky in about a month.

Jupiter, Saturn, Mars and the other planets also complete journeys around the sky in this direct, roughly eastward motion. They never stray far from the ecliptic as they journey through the constellations of the zodiac.

Yet sometimes an outer planet like Mars will stop its direct motion, and rise several nights in a row near the same position against the background of fixed stars. This is its first stationary point.

On subsequent nights it moves backwards, reversing its path in the sky. This is retrograde motion. Coincidentally, retrograde motion only occurs when the planet is opposite the Sun, visible through the entire night. The planet appears much brighter during retrograde motion than at other times. Eventually, the planet comes to another halt, which is the second stationary point.

After rising a few nights near the same position in its second stationary point, it then resumes its ordinary direct motion.

Retrograde motion demo (requires QuickTime).

Usually the Moon lies a little above or below the line between the Sun and the Earth, but occasionally it may happen to fall exactly on the line. If it does, then the Earth’s shadow will move across the face of the Moon, eclipsing the Moon.

In the late 8th century BC, during the reign of Nabonassar, the scribes of Babylon initiated an effort to observe every lunar eclipse. Despite the tumults of later conquests and revolutions, the scribes maintained records of lunar eclipses in a continuous sequence down to the first century BC.

Exhibit credit: Kerry Magruder.