An Extraordinary Satellite

In April this column covered an ordinary star, the Sun. This month lets review a most extraordinary satellite, the Moon. The Moon's mass is a bit more than 1.2% of Earth's mass. Only one other satellite, Charon, is larger than 0.005% of its planet's mass. The Charon/Pluto ratio is larger than the Moon/Earth ratio but simply because Pluto and Charon are so tiny. Together their mass is less than the Moon. The Moon is the only large satellite in the inner solar system. Venus and Mercury have no satellites at all. The Moon is more than a million times the mass of either Deimos or Phobos, the satellites of Mars. These satellites are probably nothing but captured asteroids.

The Moon follows a unique path. The Moon's orbit is jointly determined by the Earth and the Sun. From a geocentric viewpoint, the Moon seems to orbit the Earth in an elliptical path. However, seen from far above the Solar System, the Moon seems to orbit the Sun while being pulled from side to side by the Earth. The Moon maintains a prograde [moves in the direction of the planets] orbit. The satellites of outer planets simply circle their planet essentially unaffected by the Sun. The outer satellites move in both prograde and retrograde [opposite the direction of the planets] directions every orbital cycle. Drawn to scale, the Moon's orbit looks like a gently swaying curve. The orbits of outer satellites are curled tightly like spiral notebook springs.

Many of the outer satellites of the outer planets are captured comets and asteroids. These captured satellites are tiny with eccentric, highly inclined orbits. The closer satellites normally lie near their planet's equatorial plane. Our Moon behaves like neither class of satellites. The Moon is in a tug of war between the gravity of the Earth and the Sun. Adjusting for the relative mass and distance of the Sun and the Earth, the Sun pulls the Moon about twice as hard as the Earth. Nowhere else in the solar system is the tug of war so evenly balanced. Earth pulls the Moon toward the equatorial plane while the Sun pulls it toward the ecliptic plane. Since the planes are tilted, it is impossible for an orbit to be in both planes simultaneously. The Moon splits the difference by riding in its own plane balanced between these planes. It will probably come as little surprise that of the large bodies in the Solar System the Moon's orbit is the most difficult to calculate.

The Moon's composition is very similar to the rocky mantle of the Earth. This leads credence to the theory that the Moon was once part of the Earth. Most hypotheses about the Moon's formation simply do not survive scientific analysis. The few hypotheses which survive analysis are very far from being proved.

The impact of the comet Shoemaker-Levy 9 with Jupiter was spectacular. However it is probably not the first time humans have seen the impact of an asteroid or comet with a major world. During the middle ages, European monks recorded an orange red glow on the Moon's edge. There is a crater at the approximate location of the glow which appears to be very recent. From time to time, reports are made of lights, dusty clouds and gas discharges by both amateurs and professional astronomers. At least some of these reports are probably caused by impacting meteors.

The night sky is dominated by two bright planets, Venus and Mars. As always Venus, doesn't show any features except its shape. By the end of the month Venus will grow in size and show just slightly less than half of it surface. Mars is putting on quite a show. I can make out clearer surface markings than I have seen in two years. Not only are the polar ice caps visible, but the faint dusky patches of the Martian highlands are visible in my 3.5 inch telescope. We see even greater detail in Frosty Drew's 7 inch refractor. Mars should be spectacular when our 16" Schmidt Cassegrain telescope becomes operational later this year.

Leslie Coleman
Leslie Coleman
Entry Date:
May 1, 1999
Published Under:
Leslie Coleman's Columns
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