likely to have been set—by whoever was in charge of setting the calendar—on
the basis of prediction alone.
Another conclusion that emerges from this analysis is the general accuracy
of the Babylonian calendar in the Astronomical Diaries. In spite of the
relatively high incidence of one-day deviations from the new moon (early
and late, totalling about 8% of all cases), modern astronomy shows that the
new moon would always have been visible—even if in practice, not always
sighted—on either the 30th or the 31st day of the outgoing month.^64 This
contrasts with the neo-Assyrian period, where as we have seen, it was some-
times possible for the new moon to be sighted on the 28th or 29th. This means
that in the later period, the Babylonian month conformed much more accu-
rately to the new moon.
This conformity to the new moon was the direct result of increasing reliance
on new moon predictions. In the neo-Assyrian period, indeed, the possibility
of new moon sightings on the 28th or 29th was the result of excessive
dependence on new moon sightings for determining the beginning of the
month. For if the new moon was not sighted after 29 days (e.g. because of bad
weather), the beginning of the month was always postponed to the next day,
and the outgoing month thus counted 30 days. This could lead to an excessive
number of 30-day months, each time the result of bad weather, which had the
cumulative effect of delaying the month in relation to the moon. A run of 30-
day months, for example, could easily cause a subsequent new moon to
become suddenly visible on the 28th or 29th.^65 The elimination of such
fluctuations and irregularities in the period of the Astronomical Diaries—
when new moons were never visible outside the 30th or 31st of the outgoing
month—was the result of a new calendrical policy, where months were
allowed to begin even when the new moon had not actually been sighted:
long runs of 30-day months were avoided by beginning certain months when
the new moon had only been predicted. A judicious combination of new moon
sighting and new moon prediction thus maintained the Babylonian month in
a stable—and in 92% of cases, accurate—relationship with the new moon.
How exactly new moon sighting and prediction were combined can be
further established on the basis of the Diaries. If the new moon was sighted
(^64) This is because in the Astronomical Diaries early cases always occur on the 30th and late
cases always on the 31st, and these deviations are never more than of one day. My reference to
‘30th’and‘31st’days is only notional, since one of these days was always designated day 1 of the
new month. By‘visible’I mean that in astronomical terms it should have been possible to see the
new moon, even if in practice this may have been prevented by bad weather.
(^65) One text, Hunger (1992) no. 506, reports that in four consecutive months (VI, VII, VIII,
and IX) the moon had been sighted on the 1st (i.e. 31st of the outgoing month), which means a
run of four 30-day months. This occurrence could have caused a subsequent new moon sighting
on the 29th or 28th. Significantly, runs of four 30-day months are not attested in the Astronomi-
cal Diaries.
90 Calendars in Antiquity