n eclipse of the moon takes place, and this is visible at
all places where the moon is above the horizon at the time. If,
however, the moon is in conjunction, or "new," it is the sun that is
eclipsed, and as the shadow cast by the moon is but small, only a
portion of the earth's surface will experience the solar eclipse. The
nodes of the moon's orbit are not stationary, but have a daily
retrograde motion of 3' 10.64''. It takes the moon therefore 27{d} 5{h}
5{m} 36{s} (27.21222{d}) to perform a journey in its orbit from one node
back to that node again; this is called a Draconic period. But it takes
the moon 29{d} 12{h} 44{m} 2.87{s} (29.53059{d}) to pass from new to
new, or from full to full, _i. e._ to complete a lunation. Now 242
Draconic periods very nearly equal 223 lunations, being about 18 years
10-1/3 days, and both are very nearly equal to 19 returns of the sun to
the moon's node; so that if the moon is new or full when at a node, in
18 years and 10 or 11 days it will be at that node again, and again new
or full, and the sun will be also present in very nearly its former
position. If, therefore, an eclipse occurred on the former occasion, it
will probably occur on the latter. This recurrence of eclipses after
intervals of 18.03 years is called the Saros, and was known to the
Chaldeans. We do not know whether it was known to the Hebrews prior to
their captivity in Babylon, but possibly the statement of the wise king,
already quoted from the Apocryphal "Wisdom of Solomon," may refer to
some such knowledge.

Our calendar to-day is a purely solar one; our months are twelve in
number, but of purely arbitrary length, divorced from all connection
with the moon; and to us, the Saros cycle does not readily leap to the
eye, for eclipses of sun or moon seem to fall haphazard on any day of
the month or year.

But with the Hebrews, Assyrians, and Babylonians it was not so. Their
calendar was a luni-solar one--their year was on the average a solar
year, their months were true lunations; the first day of their new month
began on the evening when the first thin crescent of the moon appeared
after its conjunction with the sun. This observation is what is meant in
the Bible by the "new moon." Astronomers now by "new moon" mean the time
when it is actually in conjunction with the sun, and is therefore not
visible. Nations whose calendar was of this description were certain to
discover the Saros much sooner than those whose month