January: The Moon’s Gravity

January 2012

The Moon’s Gravity

By Calvin L. Chrisman


Full MoonIf you go outside this winter and take a look at the Moon, you will be looking at a world that has recently been joined by two satellites. These two satellites may help solve the mystery of whether there were once two moons orbiting the Earth. Yes, two moons, not one. The twin satellites, known as GRAIL (Gravity Recovery and Interior Laboratory) A and B, Went into a polar orbit around the Moon on January 1st. Since they will be in a polar orbit, the Moon will slowly rotate beneath them allowing extremely close measurements of the highly irregular gravity field of the entire Moon and, hopefully leading to some explanations of those irregularities.

Today, most experts agree that the Moon resulted from a collision between the Earth and a Mars-sized object about 4.5 billion years ago. The collision caused the ejection of a large amount of matter that eventually coalesced into the Moon. Since we have been sending men and machines to the Moon, we have learned that there are extreme differences between the physical structure and the density of the material on the Moon’s near side, which faces us, and its far side. There are a number of possible explanations for these differences, but one holds that the material ejected from the Earth coalesced into a larger moon and a smaller one, perhaps 1/3 of the size of the larger. The smaller one could have followed the larger one in what is known as a Trojan orbit, for perhaps tens of millions of years. Eventually, it would have collided with the larger infant moon in what amounts to a low-speed splat rather than a high-speed crash. This would have resulted in the two bodies accreting together rather than blowing apart and might account for the irregularities in the Moons surfaces and its interior gravity. We will see what GRAIL has to tell us.

January is a good month for planet watching. Venus is extremely bright (approximately -4.1 magnitude) and is rising higher and higher in the southwestern sky throughout the month. In a telescope, it is a gibbous disk dropping from 83% illuminated to 75% as the month progresses. Jupiter is high in the sky after sunset and shines out at -2.6 magnitude. The Moon will appear on Jupiter’s right on the 29th and on its left on the 30th. If you observe Venus and Jupiter during the month, they will move closer and closer to one another. They will eventually pass each other in the middle of March. This is something to look forward to! Mars rises by 8:30 p.m. near the end of January and is highly visible in the early morning sky.

Clear Skies!

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