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The Moon

  • The Moon is a satellite of Earth. It is not a planet.
  • The Moon has a diameter of 2,160 miles or 3,547 kilometers.
  • At an average distance of 237,100 miles or 381,550 km, 
    it takes 27 days 8 hours to circle the Earth.
  • One day lasts 27 days 8 hours of Earth time.

Quarter Moon Digital Image by Thomas Williamson,
Fort Worth, TX

About Earth’s Moon

The Earth's moon is unique in the solar system in that it is so large relative to its home planet, Earth. There are larger moons, but they are orbiting proportionally larger planets. Therefore, the origin of our moon has been the focus of considerable debate. There are three modem theories of the origin of the moon. They are:

  1. Capture Theory: The moon was captured by the gravitational field of the Earth, but this would not explain why the moon orbits the Earth in the same plane as the Earth orbits the sun. Many moons of other planets appear to be captured satellites, but they are very small and have very odd orbits.
  2. Twin Theory: The moon formed alongside the Earth during its early development. However, one would then expect the Earth and moon to be made of the same material, which they are not.
  3. Impact Theory: The early Earth was impacted by a large object, perhaps the size of Mercury. It then jettisoned a large amount of exterior material into space. This theory explains why the moon is made of mostly lighter material than the Earth, and why it is so large. The impact theory is now the most widely accepted.

The moon has several notable surface features. The first is its craters. These craters indicate that the moon does not have an atmosphere (otherwise the craters would have long ago eroded due to atmospheric conditions such as wind), and that the moon is not tectonically active. Each crater is named for a famous scientist, i.e., the Tycho Brahe crater and the Kepler crater. The dark regions on the moon are called Maria (Mare is Latin for sea), and each dark region is so named, i.e., the Sea of Tranquillity, the Sea of Storms. The Maria are made of newer, heavier material than the lighter, highland regions. They are therefore thought to be congealed lava pools created by large impacts. The highland regions are the light areas of the moon and are mostly made of anorthosite, which is a lighter but older type of rock.

The Moon and Eclipses

  1. A lunar eclipse is when the earth casts its shadow; on the moon.
  2. A solar eclipse is when the moon casts its shadow on the earth.
  3. These two types of eclipse often happen about two weeks apart, that being the time it takes the moon to travel from one side of the Earth to the other. Eclipses do not happen every month because the plane in which the moon orbits the Earth is inclined by 5 degrees' to the plane in which the Earth orbits the sun. Therefore, the shadow of the moon is usually above or below the Earth, and the shadow of the Earth is usually above or below the sun.
  4. When one type of eclipse happens, the other will usually happen soon after. This is called an eclipse season, and happens somewhere on Earth at least once a year.

Galileo Spacecraft Image of the Earth/Moon System The moon orbits the Earth every 27.322 days. This is called the moon's sidereal (with reference to the stars) period. Because the Earth is revolving around the sun, however, it takes the moon 29.53 days to go through its phases as seen from Earth. This is called its synodic (with respect to the sun) period. There are eight names for the phases of the moon. A new moon is completely dark. Next comes waxing (growing brighter) crescent (1/4 lit), first quarter (1/2 lit), waxing gibbous (3/4 lit) and full. A full moon is completely lit. After the full moon the phases are waning (growing dimmer), gibbous (3/4 lit), third quarter (1/2 lit) and waning crescent (1/4 lit).

The tides are caused mostly by the moon, and moderately by the sun. The moon's gravity pulls on the Earth, causing a bulge of water. The Earth then spins beneath this bulge of water, causing the tides. Because there is a bulge at the side of Earth facing the moon, and the exact opposite side facing away from the moon, there are two high tides and two low tides every day. When the moon and the sun are working together to make very big tides, the effect is called Spring Tides. When they are working against one another (at right angles to one another) to produce very small tides, the effect is called Neap Tides.

Moon/Venus photo shot with a 200mm lens, 100ASA Focal color film,
Mayetta, Kans. 05/18/99.

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