MadSci Network: Astronomy Query:

### Re: If the moon was moved 128,000 km closer to the sun, what would happen?

Date: Sun Mar 9 18:28:03 2003
Posted By: Pauline Barmby, Astrophysicist
Area of science: Astronomy
ID: 1041816048.As
Message:

Dear Mary,

Let me first say that moving the moon would be a difficult job: to seriously affect the orbit of an object you have to hit it with something that is either close to its own mass or moving very, very fast. The moon is about a hundred times more massive than the biggest asteroid in the solar system, and the asteroids aren't generally moving incredibly fast. Also, hitting the moon with something would probably transfer energy to the moon, which would result in its orbit getting larger (i.e., further from the Earth).

But let's say it was somehow moved closer to the Earth. 128000 km is about a third of the radius of the moon's current orbit. Using Kepler's formula, `p3/t2=constant`, we can compute that the period of the moon's orbit would become shorter, around 22 days. So the time between full moons would be a little over 3 weeks. If the moon was still rotating at the same speed it does now, then we would see more of it: now we only see one side because the periods of rotation and revolution are the same.

If the moon were closer, it would be look bigger in the sky: if it were only 2/3 as far away, then it would look 1 1/2 times as big as it does now. The total amount of light we see from the moon would increase by 25%, but how bright any piece of the surface appears would be the same as before (because the larger amount of light is also spread out over a larger area).

How would the Earth be affected? As far as social effects, one group of people who currently pay attention to the moon are astronomers: both those who want to observe the moon when it's up, and those who want to observe when the moon is not up (it affects the brightness of the night sky and makes it difficult to see faint things). Another group of people who pay attention to the moon are Muslims, since the Muslim month begins when the crescent moon is sighted. So the schedules of both astronomers and Muslims would change. Most other folks don't pay that much attention to the moon, so I doubt they would care or even notice.

How would the Earth be affected geologically? Well, the tides caused by the moon would be stronger if the moon were closer, so there might be more erosion and other tidal effects. There are actually small "solid Earth" tides (the Earth itself has tides of a few centimeters), so these would also be stronger. I am not a geologist, but I doubt these would have any serious effect on the Earth as a whole.

I've given a few ideas on how changing the size of the moon's orbit would affect the Earth; it might be a fun project for you to try and think of some more. If you're interested in "what if the moon weren't there?" there are several answers to that question in the MadSci archives.

Pauline

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