predicted at any hour, their
course in the firmament being traced so accurately that its various
stages were marked out and indicated beforehand. The moon, they
discovered, had to complete two hundred and twenty-three revolutions of
twenty-nine days and a half each, before it returned to the point from
which it had set out. This period of its career being accomplished, it
began a second of equal length, then a third, and so on, in an infinite
series, during which it traversed the same celestial houses and repeated
in them the same acts of its life: all the eclipses which it had
undergone in one period would again afflict it in another, and would
be manifest in the same places of the earth in the same order of time.*
Whether they ascribed these eclipses to some mechanical cause, or
regarded them as so many unfortunate attacks made upon Sin by the seven,
they recognized their periodical character, and they were acquainted
with the system of the two hundred and twenty-three lunations by which
their occurrence and duration could be predicted. Further observations
encouraged the astronomers to endeavour to do for the sun what they had
so successfully accomplished in regard to the moon.

* This period of two hundred and twenty-three lunations is
that described by Ptolemy in the fourth book of his
"Astronomy," in which he deals with the average motion of
the moon. The Chaldaeans seem not to have been able to make a
skilful use of it, for their books indicate the occurrence
of lunar eclipses outside the predicted periods.

No long experience was needed to discover the fact that the majority of
solar eclipses were followed some fourteen days and a half after by an
eclipse of the moon; but they were unable to take sufficient advantage
of this experience to predict with certainty the instant of a future
eclipse of the sun, although they had been so struck with the connection
of the two phenomena as to believe that they were in a position to
announce it approximately.* They were frequently deceived in their
predictions, and more than one eclipse which they had promised did not
take place at the time expected:** but their successful prognostications
were sufficiently frequent to console them for their failures, and to
maintain the respect of the people and the rulers for their knowledge.
Their years were vague years of three hundred and sixty days. The twelve
equal months of which they were composed bo