The Moon is the Earth’s closest neighbor in space.  The average distance of the Earth and the Moon 238,857 miles (384,403 km), (Burgess, 1993). It takes just 6 days to get to the Earth and back in a rocket.   Because the Moon is relatively close to the Earth it seems much larger than the stars and about the same size as the sun.   The moon measures about 2,160 miles across.  The distance is about one-fourth the diameter of the earth and 400 times smaller than the Sun, (Scientific American).  Compare the Moon with your favorite satellite or planet.

The Moon travels at an average speed of about 2,300 miles per hour along 1.4 million-mile (2.3 million-km) orbit.  It completes one revolution in about 27 days, 7 hr, 43 min, and 11.5 seconds with reference to the stars (sidereal month).  It takes 29 days, 12 hr, 44 min, and 2.8 seconds for the moon to complete one cycle of lunar phases, and thus is measured with respect to the sun.  The moon rotates on its axis once in about the same period of time that elapses for its sidereal period of revolution, accounting for the fact that virtually the same portion of the moon is always turned toward the Earth.  Although the moon appears bright to the eye, it reflects into space only 7% of the light that falls on it.(1).

 The Moon has no atmosphere.  An early atmosphere that the Moon might have had has escaped from the Moon’s feeble gravitational pull.  As a result of its lack of atmosphere, the moon has no weather, clouds, no rain, and no wind.  At night, the rocky surface becomes colder than any place on Earth.  In the day, the temperature of the rocks is slightly higher than that of boiling water.(2).

 The surface of the moon is much hotter and much colder than any place on Earth.  At the moon’s equator, noon temperatures on the maria are as high as 260 degrees F (127C).  Temperatures drop below
-280 degrees F (-173C) during the two week lunar night.  In some deep craters near the Moon’s poles, the temperature is always near –400 degrees F.(2).
 

Most of the maria were formed from 3.3 to 3.8 billion years ago by great flows of lava that poured
Out and cooled on the Moon’s surface.  The lava that formed the maria has filled in the low places on the moon.  Some of these low places are giant craters.(3)
 

 Craters are the most numerous features on the moon’s surface.  The Moon has craters within craters and even connected craters.  Scientists estimate that the Moon has 500,000 craters that are more than one mile wide.  A total of about 30 thousand billion craters are at least 1 foot wide.  Earth by comparison has only 200 known impact craters.(3).
 

Billions of small craters on the Moon have been formed by the impact of meteoroids.  Many
Meteoroids also hit the Earth’s atmosphere, most melt or break up high in the air.  Only the largest meteoroids reach the Earth’s surface fast enough to form a crater.  The moons lack of atmosphere means that even tiny meteoroids form craters.  Erosion on the Moon works so slowly that craters only 1 foot in diameter remain for millions of years.(2).

The moon also has long narrow valleys called rilles.  Most rilles are straight and probably were formed when the outer crust was cracked, and sections of the surface dropped down.

Origin of the Moon

 The earth and the meteorites that have fallen to earth have been dated some 4.6 billion years ago.  Largely as a result of this evidence, most scientists assume that the entire solar system was formed at about the same time.  However, none of the lunar riocks brought back to earth by the Apollo missions has proved to be more than 4.2 billion years old.(2).

        **In 1879, George H. Darwin suggested that the earth and the moon were once a single body.  Shortly after the earth was formed, according to his theory, a huge buldge was produced on the earth by the attraction of the sun.  The earth was spinning much more rapidly than it is today, and the bulge broke away and formed the moon.  This theory does not explain why the moon lacks the dense elements compared to earth.(4).

       **A second theory called the capture theory, states that the moon was formed as a seperate planet that followed its own orbit around the sun.  It states that the moon was adventually drawn into orbit by the Earth’s gravitational forces.(4).

      **A third theory is that the Earth and the moon were formed close to each other from a disk of gas around the sun.  They were formed as a double-planet system, much like the systems of double stars.

     **A fourth theory that was first proposed in the 1980’s is called the collision ejection theory.  This proposes that the earth was struck by a mars sized object in the first million years after our planet formed.  The collision blew material from the earth’s mantle into outer space.  This material then adventually united into a single body to form the moon.  This is a widely accepted theory.  This helps to explains why the rocks and other materials found on the moon are similar to those found on earth.(4).

Ancient History

 Since ancient times, people have measured time by the phases of the moon.  There are man-made calenders that have  been dated back 30,000 years ago.  There has been archialogical finding from the ice age where hunters carved notches and holes into sticks andreindeer bones depicting the days between each phase of the moon.  These artifacts are dated between 25,000 and 10,000 B.C.(7).
 
 The first calenders that were developed around the lunar cycle.  Back in Egyptian times they calculated that a month was the period the Moon revolved around the Earth, and from this understandingvarious lunar calenders evolved.  Until the times of Julius Caesar, the calender was primarily lunar.  Many Muslim countries still use a calender with 354 days, or 12 synodic lunar months.  The lunar calender is still used by the Jewish to establish dates of religious Holidays.  Christians observe Easter on the first Sunday  following the first full moom in April.(7).
 

Exploration

From 1969-1972 12 astronauts walked on the lunar surface.  While there these astronauts left artifacts of our most advanced technology, including Lunar Rovers, and science equipment.  The equipment was designed to radio reports of moonquakes and meteorite impacts back to Earth.  The Apollo program died before the program was complete.  Apollo's 18, 19, and 20 would have continued to use our growing confidence to reach more forbidden areas including large craters and the highlands, as well as a mission to the far side.  Our brief exploration has raised many questions.  Does the Moon have a molten core?  Why does the farside have a thicker crust than the near side?  What are the origins of the Moon's hot spots -are they potential gas wells leading down to a still active interior?  We cannot know until we return to the Moon.  For a complete look at Moon exploration, see Exploring the Moon. (6).