The moon is the only natural satellite of Earth. The moon orbits the Earth from
384,400 km and has an average speed of 3700 km per hour. It has a diameter of
3476 km, which is about ¼ that of the Earth and has a mass of 7.35e22 kg. The
moon is the second brightest object in the sky after the sun. The gravitational
forces between the Earth and the moon cause some interesting effects; tides are
the most obvious. The moon has no atmosphere, but there is evidence by the
United States Department of Defense Clementine spacecraft shows that there maybe
water ice in some deep craters near the moon's North and South Pole that are
permanently shaded. Most of the moon's surface is covered with regolith, which
is a mixture of fine dust and rocky debris produced by meteor impact. There are
two types of terrain on the moon. One is the heavily cratered and very old
highlands. The other is the relatively smooth and younger craters that were
flooded with molten lava. Throughout the 19th and 20th centuries, visual
exploration through powerful telescopes has yielded a fairly comprehensive
picture of the visible side of the moon. The hitherto unseen far side of the
moon was first revealed to the world in October 1959 through photographs made by
the Soviet Lunik III spacecraft. These photographs showed that the far side of
the moon is similar to the near side except that large lunar maria are absent.
Craters are now known to cover the entire moon, ranging in size from huge,
ringed maria to those of microscopic size. The entire moon has about 3 trillion
craters larger than about 1 m in diameter. The moon shows different phases as it
moves along its orbit around the earth. Half the moon is always in sunlight,
just as half the earth has day while the other half has night. The phases of the
moon depend on how much of the sunlit half can be seen at any one time. In the
new moon, the face is completely in shadow. About a week later, the moon is in
first quarter, resembling a half-circle; another week later, the full moon shows
its fully lighted surface; a week afterward, in its last quarter, the moon
appears as a half-circle again. The entire cycle is repeated each lunar month,
which is approximately 29.5 days. The moon is full when it is farther away from
the sun than the earth; it is new when it is closer. When it is more than
half-illuminated, it is said to be in gibbous phase. The moon is waning when it
progresses from full to new, and waxing as it proceeds again to full.
Temperatures on its surface are extreme, ranging from a maximum of 127° C (261°
F) at lunar noon to a minimum of -173° C (-279° F) just before lunar dawn. The
Harvest moon is full moon at harvest time in the North Temperate Zone, or more
exactly, the full moon occurring just before the autumnal equinox on about
September 23. During this season the moon rises at a point opposite to the sun,
or close to the exact eastern point of the horizon. Moreover, the moon rises
only a few minutes later each night, affording on several successive evenings an
attractive moonrise close to sunset time and strong moonlight almost all night
if the sky is not clouded. The continuance of the moonlight after sunset is
useful to farmers in northern latitudes, who are then harvesting their crops.
The full moon following the harvest moon, which exhibits the same phenomena in a
lesser degree, is called the hunter's moon. A similar phenomenon to the harvest
moon is observed in southern latitudes at the spring equinox on about March 21.