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On the Babylonian Sighting-Criterion of the Lunar Crescent and its implications for Egyptian lunar data
Journal
The Journal for the History of Astronomy
Volume
45
Number
1
Pages / Article-Number
79-96
Keywords
sighting criterion, lunar crescent, Babylonia, Egypt, lunar data
Abstract
The prediction of the first or last visibility of the lunar crescent is a non-trivial task. Hence, the solution of this problem has attracted much attention without being solved definitely as of today. Most ancient civilisations in the Near East used a lunar calendar with a new month starting at first or last visibility of the lunar crescent. Thus the prediction of first or last visibility is indispensable for comparing ancient astronomi- cal observations or predictions with today’s calculations for dating purposes. From information preserved on cuneiform tablets it is clear that the Babylonians were in possession of a sighting-criterion for the Moon as the tablets contain not only observed but also predicted first or last crescents.
Babylonian lunar observations and predictions are a valuable source of information not only for investigations of Mesopotamian astronomy, but also for Egyptian chronology. Due to the complete lack of systematic records of astronomical observations in ancient Egypt, no visibility lines based on Egyptian data alone can be obtained for Egyptian lunar observations. Besides modern astronomical observations, only the reference material from Babylon allows us to establish ancient visibility criteria and thus to use for dating purposes the 100 or so documented lunar feast dates prior to Graeco-Roman times in Egypt. Usually, the astronomical dating of pharaohs does not provide a unique solution but leaves two or more options. The “correct” option has then to be chosen tentatively, so that various authors have different opinions on the “correct” chronology.One of the most important questions for choosing the “correct” chronology is how to deal with presumed deviations in a positive direction, i.e. with records documenting a sighting of the lunar crescent when according to calculations the Moon should have been already invisible. Such a record may be a knock-out criterion for an otherwise perfectly fitting chronological option.
The aim of the present paper is to establish a lower boundary condition of a possible successful sighting of the lunar crescent based on Babylonian predictions and observations. This means that all Egyptian chronological options showing deviations in positive direction with visibilities below this boundary condition will be dismissed as impossible. Although a theoretical limit of 10° for the visibility criterion, i.e. a minimum lagtime between sunset and moonset of 40 minutes, is mentioned in text TU 11, the Babylonian Astronomical Normal Star Almanacs contain several predictions for first or last visibility with values smaller than 10° and the Astronomical Diaries a few such observations. Thus, a value of 10° for the lagtime is an inappropriate lower boundary condition which cannot be used as knock-out criterion for Egyptian chronological options based on lunar data.
