of these ever-moving bodies will come into one and the same straight
line. Now the consequences of this state of things were admirably well
pointed out nearly half a century ago by a popular writer, who in his
day greatly aided the development of science amongst the masses. "When
one of the extremes of the series of three bodies which thus assume a
common direction is the Sun, the intermediate body deprives the other
extreme body, either wholly or partially, of the illumination which it
habitually receives. When one of the extremes is the Earth, the
intermediate body intercepts, wholly or partially, the other extreme
body from the view of the observers situate at places on the Earth which
are in the common line of direction, and the intermediate body is seen
to pass over the other extreme body as it enters upon or leaves the
common line of direction. The phenomena resulting from such
contingencies of position and direction are variously denominated
_Eclipses_, _Transits_, and _Occultations_, according to the relative
apparent magnitudes of the interposing and obscured bodies, and
according to the circumstances which attend them."[1]

The Earth moves round the Sun once in every year; the Moon moves round
the Earth once in every lunar month (27 days). I hope everybody
understands those essential facts. Then we must note that the Earth
moves round the Sun in a certain plane (it is nothing for our present
purpose what that plane is). If the Moon as the Earth's companion moved
round the Earth in the same plane, an eclipse of the Sun would happen
regularly every month when the Moon was in "Conjunction" ("New Moon"),
and also every month at the intermediate period there would be a total
eclipse of the Moon on the occasion of every "Opposition" (or "Full
Moon"). But inasmuch as the Moon's orbit does not lie in quite the same
plane as the Earth's, but is inclined thereto at an angle which may be
taken to average about 5-1/8 deg., the actual facts are different; that is to
say, instead of there being in every year about 25 eclipses (solar and
lunar in nearly equal numbers), which there would be if the orbits had
identical planes, there are only a very few eclipses in the year, never,
under the most favourable circumstances, more than 7, and sometimes as
few as 2. Nor are the numbers equally apportioned. In years where there
are 7 eclipses, 5 of them _may_ be of the Sun and 2 of the Moon; where
there are only 2 eclipses, both _must