These early scholars recorded their findings on thousands of clay tablets belonging to both the Babylonian and Assyrian cultures. The tablets, which have been excavated in various parts of ancient Mesopotamia - primarily Sumer - have been difficult to decipher. However, the perusal of these tablets has resulted in many important disclosures useful to the present day.
In the Babylonian civilization, both astronomy and astrology were the provinces of the priests. The society's rigid caste system restricted education. Therefore the opportunity to study and build on existing knowledge fell to this upper class. In their professional work, the priests' religious activities included prognostications in both astronomy and astrology. In later Mesopotamian civilization, the priests began to move away from the pure science of astronomy and into the more vernacular astrology, often losing the clarity of their earlier work. Likewise, the Chaldeans never made the progress in astronomy eventually achieved by the Greeks. Still, they are credited with some discoveries which predate their Greek counterparts.
Much of the data was collected before the era of Nabonassar, the time period which occurred after the destruction of the Assyrian capital city of Nineveh in the Seventh Century B.C.. The historical records accumulated over the course of many centuries before the destruction of Nineveh made it possible to verify and establish short-range astronomical predictions.
The information found on the clay tablets includes both observations and calculations of the motions of the planets. The Chaldeans did less with astronomy than the Babylonians. Nevertheless, one important Chaldean discovery was a method of predicting, within a certain degree of accuracy, the apparent motion of the planets as they sped through the sky. These predictions included times of retrograde (backwards) motion, helical rising and setting, and conjunctions with principal stars. The Chaldeans were also able to calculate the times of Earth's Moon's "new" phase as well as lunar and solar eclipses.
Babylonian astronomy can be divided into two periods: The ancient period extending from antiquity to the destruction of Nineveh in the year 607 B.C. and the second period, beginning with the end of Nineveh to 1 B.C.. Very little is known about the first period. During that time, Mesopotamian astronomy was poorly differentiated from astrology. The priests used their knowledge as a tool in practice of their religion. However, by the second period, astronomy as a pure science had begun to take form.
Some of the oldest astronomical artifacts are astrolabe tablets. These clay tablets consisted of three concentric circles divided by twelve radii into twelve sections. Each of the thirty-six fields contained the names of constellations and simple numbers. No one yet understands the significance of these numbers. It is believed that the numbers represented the months of the Babylonian calendar. These calendars are similar to calendars developed by the Egyptians. Astrolabes are still used today to determine the relative positions of stars and planets.
The Babylonian calendar was lunar and based on the New Moon phase, i.e. numbered according to the days elapsed after the Moon reappeared from the end of its waning cycle. Because the seasons were governed by a solar calendar, the Babylonians derived a luni-solar calendar which consisted of twelve months, with a thirteenth one added occasionally to keep the years consistent. Some extant calendrical records also show the presence of a seven-day week system. It varied from place to place throughout Babylon. The final day, with the seventh, fourteenth, twenty-first, and twenty-eighth of each month were considered "cursed". As such, many activities were suspended on these days, and the astronomers-astrologers created rituals to ward off the supposed evil of those dates.
It is from the Babylonian astronomers that the divisions for the 360 degrees of a circle comes. The Babylonians divided the day into twelve intervals called "kaspu". The solar kaspu was the span of thirty degrees which the Sun travels in two hours of daily motion across Earth's sky. The Babylonians used the arc of one degree as a unit of angular space, just as is done today. They expanded their system to include units of time which correspond to four modern minutes, which is approximately the path taken by Earth in one day of its yearly revolution around the Sun.
The Babylonians also predicted certain celestial phenomena, such as eclipses and lunar periods. They began their studies with the eclipse of March 19, 721 B.C.. Calculations were difficult because the astronomers had no instruments of high accuracy. Both the Chaldeans and Babylonian eclipse records are used in studying long-term variations in the lunar orbit in modern theories.
Solar eclipses were the most difficult for these ancient cultures to predict. Because the method of observation was to establish lookers at certain points to watch over a specific period of time, one observer might miss five or six solar eclipses due to cloudy weather and poorly predicted locations and times. The primary predictive activity, however, was to determine the time of the New Moon phase to set their calendar.
The priests used the predictions of eclipses and New Moons to create astrological reports for the kingdom. In Babylon, astrological tables often mention the five major planets. For example, a Babylonian priest might predict: "On the twenty-fifth day, there will be a lunar eclipse; the Syrian kingdom will face grave danger; and Venus will not be seen."
While Babylonian and Chaldean astronomers were
more advanced than scholars in other contemporary cultures, they did lack
knowledge of geometry and trigonometry. This kept them behind the Greeks
in their astronomical studies. As we have seen, though, they contributed
much to the study of astronomy which has survived into the present era.
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