eclipse Babylonianastronomy

Until the discovery of the late Babylonian astronomical texts in the latter half of the 19th century, the Alexandrian astronomer Ptolemy’s Almagest (2nd century ad) was the only source of Babylonian eclipse observations. Ptolemy cites several records of lunar eclipses, the earliest in 721 bc. Unfortunately, the dates and observational details are not in original form but have been edited, presumably by the Greek astronomer Hipparchus (2nd century bc). Dates have been converted to the Egyptian 365-day calendar, while times have been expressed in hours instead of the original units.

The discovery and decipherment of vast numbers of cuneiform astronomical texts at the site of Babylon in the 1870s and ’80s completely revolutionized the study of Babylonian astronomy. Most of the extant texts, dating from about 747 bc to ad 75, are in the British Museum. Numerous day-to-day astronomical diaries contain records of celestial phenomena, including many eclipses. Although most of the tablets are very fragmentary, additional Babylonian collections of eclipse reports—abstracted from the original diaries—also survive. An example of a lunar eclipse record, dating from 80 bc, is as follows. Time intervals, presumably measured with the aid of a water clock, are expressed in UŠ (time-degrees, equal to four minutes), while eclipse magnitudes are expressed in “fingers,” each equal to ¹/₁₂ of the lunar diameter:

Year 168 [Arsacid dynasty], that is year 232 [Seleucid kingdom]…month I, day 13…lunar eclipse…. In 20 deg of night it made six fingers. Duration of maximal phase 7 deg of night, until it began to become bright. In 13 deg…4 fingers lacking to brightness it set…. [Began] at 40 deg before sunrise.

The date, when converted to the Julian calendar (April 11, 80 bc), is exactly correct. Sunrise on this occasion would occur at 5:37 am, so that the measured start of the eclipse was at 2:57 am, and maximum phase (when half of the Moon was estimated to be in shadow) was at 4:31 am. When the Moon set, one-third of its disk was observed to be still in shadow. Use of “fingers” to express the magnitude of an eclipse (both lunar and solar) spread to Greece and hence to the Arab world. This convention was still fairly standard among astronomers worldwide until the 20th century.

Babylonian eclipse predictions, which were based on past series of observations, were fairly accurate for this early period. Timing errors averaged about two hours, and predictions gave a useful indication of the likelihood of an eclipse to intending observers.