A satellite of the Earth, which to different civilizations has also been known as Luna, Soma, Isis; the "mother
of the Earth." It has given us the name for the first day of the week-Monday; also lunacy, lunatic, moonstruck.
The Moon, reflecting the light of the Sun, emits a degree of heat which can be registered by concentrating the
rays on the bulb of a thermometer. It may have some slight vegetation, but because of the apparent absence of
atmosphere or clouds it lacks sufficient water to support vegetation such as is on the Earth.
The period of the Moon's axial rotation is the same as its period of revolution, hence the same side of the Moon
is always turned toward the Earth. That its orbit was formerly smaller and its velocity correspondingly greater
is proved by comparing records of ancient eclipses to tables based on observation of its present motion. The
Moon's mean distance from the Earth is 238,840 miles, or 60 times the Earth's radius. It travels a trifle faster
than its diameter per hour. Nor is it entirely the nearest body to the Earth, for in part of its orbit the minor
planet Hermes (disc. in 1937) approaches to a distance of only 200,000 Miles. Traveling by airplane at 200 m.p.
h. one would traverse the Earth-Moon distance in 5o days; but it would take a rocket ship speed of 7 m.p.s. to
get beyond the Earth's gravitational field-at which rate we could arrive in 2 days.
Lifetimes have been devoted to the study of its incredibly complex motions. Among its various perturbations
are the Equation of the Center, the retrogression of the Nodes, Evection, the anomalistic period, Lunar
Variation, Annual Equation, and Secular Acceleration.
Galilee, in 1610, was the first selenographer to study the Moon through a telescope. In 1647 Hevelius
published a chart of the Moon's surface that was not improved upon for a century. Its phases are familiar: The
crescent of the new moon, and the reverse crescent of the fourth quarter of its circuit; the gibbous phase of the
second and third quarters, when more than half of the moon is light; and the Earth-shine, when the Earth
reflects a dim light upon the surface of the Moon during a few days before and after the Lunation.
Because of its faster motion near perigee we are able to see 7°45' around the Eastern and Western edges. This
is termed its Libration in Longitude. Because of the inclination of the plane of the Moon's orbit to that of the
Earth, we are able at times to see 6°41' beyond each of the poles. This is termed Libration in Latitude. There is
also a Diurnal Libration of 1° on the Eastern limb of the Moon when rising, and on the Western when setting.
The net combined result is that 41% of the Moon's surface is visible all the time, with another 18% that is
visible part of the time, leaving 41% that has never been seen from the Earth.
Meton discovered the recession of the Moon's node in 432 B.C. and reformed the calendar in accordance
therewith. He determined that there were 235 synodic periods in 19 years, varying by i day according to the
number of leap years contained in the period.
The node recesses 360° in 6793.5 days or 18 2/3 years, or roughly 1½ years to a sign.
The Draconitic period of the Moon's motion, that from node to node, is 27.2122 days.