Is the Moon moving away from the Earth?

Question posed by Robin

The Moon is currently 384467 km away from the Earth, and measurements show that this distance is increasing by about 3.8 cm every year.


How do we know?
I, Luc Viatour, via Wikimedia Commons
The distance from the Earth to the Moon is currently measured by bouncing laser light off special reflectors that were left there by astronauts on the Apollo 11, 14 and 15 lunar missions. We know how fast laser light travels (light speed, funnily enough!) and we can time how long it takes for a beam of light to get to the Moon and back. If you know how fast something's travelling and how long it takes to get there, you can work out how far it went quite easily (you've probably heard that 'speed equals distance over time'!)


Why is the Moon moving away from us?
It's not because we smell.

You are probably aware that the Earth holds on to the Moon by a force called gravity. What many people don't realise is that the Moon also attracts the Earth with its gravity: it's a mutual thing. You can see one of the effects that the Moon has on Earth when you go to the seaside- over the course of a day, the sea rises and falls (we call this a tide). Sea tides are caused by the water on the surface of the Earth being attracted by the Moon's gravity, and actually being dragged into a bulge that rises above the Earth's surface.

The Wetzell Lundar Laser Ranging Station, By H. Raab, via Wikimedia Commons
Whilst all this is happening, the Earth is rotating. Friction between the Earth's surface and the ocean means that the tidal bulge caused by the Moon's gravity is dragged around the Earth, and the bulge ends up slightly ahead of the Moon, rather than directly below it.

Now, because the Earth's oceans are made up of water, and water has mass, the water itself exerts a gravitational pull on everything else, including the Moon. Just as the water is dragged around by the Earth's spin, the water pulls the Moon sideways a little in its orbit. This means that the Moon actually speeds up a tiny bit in its orbit around the Earth.

As the Moon starts to move faster, its orbit becomes wider, and it moves a little bit further away from the Earth.


Is this speeding up or slowing down?
Gravity has less of an effect the further away you are, so as the Moon recedes from the Earth, the effect that each body has on the other reduces slightly. Also, as the Earth's rotation is slowed by tidal effects, the ocean tides aren't pulled as far or as fast in front of the Moon, so the effect of this is lessened. So, in the long term, the Moon's journey will be slowed, but such changes take too long to measure on any human timescale.


What other effects does this have?
To speed something up or change its orbit you need energy, and this has to come from somewhere. In this case, it comes from the rotation of the Earth. This means that as the Moon's orbital distance is increased, the Earth's rotation slows down a little. Currently, the Earth's day is becoming shorter by about two milliseconds every hundred years.

Although we quite often use the word 'tides' to refer to the effect of the Moon on bodies of water on the Earth, tidal forces actually effect the whole of a planet or satellite. Even on the Earth the effect of the Moon is measurable, with the Earth's rocky surface being squeezed or stretched a number of centimetres by the effect of the Moon's gravity.


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Comments

  1. I don't smell, but the moon people tell me you do a little bit! ;-)

    ReplyDelete
  2. Global Warming is caused by the earth's orbit around the sun is destabilizing, and decaying. Greenhouse gases are not to blame, you have been lied too.
    http://orbital-decay1.blogspot.com

    ReplyDelete
  3. Thanks for your comment, Willie. Unfortunately, I didn't read past the second paragraph of the information at the link you posted, as what I did read was riddled with misconceptions, inconsistencies and outright falsehoods. It seems that the author doesn't understand at all the forces of gravity and electromagnetism.

    I wonder if, for my readers, you would be so kind as to summarise details such as the mechanism causing the 'decay' of the Earth's orbit, and the evidence which leads you to this conclusion?

    Thanks again!

    ReplyDelete
  4. If Willie's science is as good as his grammar, it can safely ignored.

    ReplyDelete
  5. Good grief: "The aurora, and australis borealis is responsible for our weather, thunderstorms, snowstorms, hurricanes, tornadoes, and wind."

    Um... no. Really, no. I'm curious to know what the australis borealis is, too: I'm no latinologist, but it looks like it means 'northern southern'.

    Haha: "The earth has tilted on its axis 26.0 degrees beyond normal, and the tilting of the earth axis is worsening" -- and nobody noticed the Arctic circle move! How negligent of us.

    Perhaps a good book would be to go through this sort of ridiculous bunk and give it a good fisking, which isn't as dirty as it sounds.

    ReplyDelete
  6. Precisely: Come for the lack of capital letters beginning proper nouns; stay for the blatant disregard of scientific methods and understanding!

    It's too easily debunkable for a book: "This bit is wrong. Ref: Ladybird Book of Aurorae. This bit is also wrong. Ref: Ladybird Book of Gravity. This bit is wrong, too. Ref: Ladybird Book of Orbital Mechanics. This bit, perhaps surprisingly, is also gratuitously wrong. Ref: Oxford English Dictionary."

    ReplyDelete
  7. Hm. Embarrassing that I have a grammatical error in my comment about Willie's grammatical errors. As a scientist, though, I own up to my errors; it should read "safely be ignored."

    ReplyDelete

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