The Solar System and the Planet Earth 297
axis pointing towards the Earth. The mass of the Moon is slightly greater
than 1% of the Earth’s mass. Despite its small mass and great distance
from Earth, its gravitational effect is felt on Earth. The rise and ebb of
the tides is due to the gentle pull of the Moon upon the oceans.
The light from the Moon like all the other objects of the Solar System
is due to the reflection of the Sun’s rays. The waxing and waning of the
Moon’s phase is due to the fact that different areas are illuminated as the
Moon orbits the Earth each month. A new moon occurs when the Moon
is between the Earth and the Sun. A full moon occurs when the Earth is
between the Sun and the Moon and the Sun fully lights the half of the
Moon facing us. From time to time during the time of a new moon the
shadow of the Moon is cast upon the Earth causing a solar eclipse.
Occasionally at the time of a full moon the Earth casts its shadow on the
Moon in which case we have a lunar eclipse.
The Moon has no atmosphere because its gravitational field is too
weak to retain any gases. The Moon also has no magnetic field. The
Moon appears to be composed primarily of rocks as indicated by its
density of 3.3 grams per cubic centimeter. The rocks are basically
basalts, which resemble the molten volcanic rock, found on Earth. The
Moon apparently formed at the same time as the Earth and passed
through a stage in its early history when its surface was covered with
molten rock.
The most outstanding feature of the lunar surface is the maria or seas
as they were first called because of their resemblance to oceans. These
dark patches on the surface of the Moon are responsible for the
appearance of the Man in the Moon. In actuality the maria are vast planes
strewn with boulders and pock-marked with small meteoric craters. The
most dramatic features of the Moon are enormous meteoric craters up to
60 kilometers in diameter. The Moon also has a number of geological
features found on Earth such as mountains and volcanic craters.
Mars
Mars is the best studied of all the planets because of its proximity and
transparent atmosphere. It shows a variety of markings, which have
fascinated man for centuries. Mars, which appears red to us, is 1.52 AU
from the Sun. It orbits the Sun every 687 days and rotates on its axis
once every 24 hours and 37 minutes. Its radius is only half that of Earth
and its density is less than ours so that its mass is only 11% that of Earth.