317. If the Moon’s Orbit were coincident with the Plane of the Ecliptic,
in which the Earth always moves and the Sun appears to move, the Moon’s
shadow would fall upon the Earth at every Change, and eclipse the Sun to
some parts of the Earth. In like manner the Moon would go through the
middle of the Earth’s shadow, and be eclipsed at every Full; but with
this difference, that she would be totally darkened for above an hour
and half; whereas the Sun never was above four minutes totally eclipsed
by the interposition of the Moon. But one half of the Moon’s Orbit, is
elevated 5-1/3 degrees above the Ecliptic, and the other half as much
depressed below it: consequently, the Moon’s Orbit intersects the
Ecliptic in two opposite points called _the Moon’s Nodes_, as has been
already taken notice of § 288. When these points are in a right line
with the center of the Sun at New or Full Moon, the Sun, Moon, and Earth
are all in a right line; and if the Moon be then New, her shadow falls
upon the Earth; if Full the Earth’s shadow falls upon her. When the Sun
and Moon are more than 17 degrees from either of the Nodes at the time
of Conjunction, the Moon is then too high or too low in her Orbit to
cast any part of her shadow upon the Earth. And when the Sun is more
than 12 degrees from either of the Nodes at the time of Full Moon, the
Moon is too high or too low in her Orbit to go through any part of the
Earth’s shadow: and in both these cases there will be no Eclipse. But
when the Moon is less than 17 degrees from either Node at the time of
Conjunction, her shadow or Penumbra falls more or less upon the Earth,
as she is more or less within this limit. And when she is less than 12
degrees from either Node at the time of opposition, she goes through a
greater or less portion of the Earth’s shadow, as she is more or less
within this limit. Her Orbit contains 360 degrees; of which 17, the
limit of solar Eclipses on either side of the Nodes, and 12 the limit of
lunar Eclipses, are but small portions: and as the Sun commonly passes
by the Nodes but twice in a year, it is no wonder that we have so many
New and Full Moons without Eclipses.
[Sidenote: Fig. I.