29. Earliest date
The impressive scope of the Grand Saros Canon and of the Saros scheme in general does not insure the validity of individual entries. The four earliest dates are computed from the text of tablet shard LBAT 1413. The preserved text is insufficient to determine a unique set of dates for these eclipses. At least one historian hesitated to ratify Feb 6, 747 BC as the earliest date recorded in a Saros.
19. As the crow flies
Information about dates from early in the Nabonassar Era has come down to us mostly from Greek sources. No tablet inscribed with the name Nabu-nasir or his immediate successors has been absolutely dated. For direct evidence of the period historians turned to Babylonian records of lunar eclipses.
20. Loony
The moon shifts rapidly across a background field of stars, about ½ degree an hour. Hence errors in calculation of the moon's position come to light expeditiously, and they are readily noticeable to astronomers observing the sky and to historians checking ancient records.
The moon's advantageous features are counterbalanced by the complexity of its orbit. Astronomers since Babylonian times have struggled to discover and describe the cyclical patterns traced by the moon. Thousands of observations of the moon in cuneiform texts attest to the close attention paid to lunar comings and goings.
21. Alignments
Eclipses are a boon to astronomers. The lineup of sun, moon and earth provides a reference that astronomers can use to test and refine their calculations.
Lunar eclipses offer additional benefits to historians. The eclipses are visible over the whole night-side of the globe and ancient peoples diligently logged them. A record of an eclipse can be used to peg a relative chronology to an absolute date.
22. The Saros period
Avid sky-watchers like the Babylonians had no difficulty working out the rudimentary pattern of lunar eclipses. They occur every 5th or 6th month, discounting about 1/3 of eclipses that occur entirely in daylight hours.
A more elusive pattern operates on a lengthy cycle of 223 months. This period of about 18 years is known as a Saros. (The term is loosely applied to a cycle of eclipses separated by 18 years, perhaps better labeled a Saros cycle.) The eclipses of a Saros cycle share a striking feature - they look the same. Hence the timing and prognostic characteristics of an impending eclipse can be determined by checking earlier eclipses of its Saros cycle.
![caeno.org](/images/N54a.jpg)