Planetary Science
The Moon
Planet Earth

Is the moon moving away from earth?

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April 12, 2013 12:56AM


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As the result of Earth's spin and it's effect on the tide, the moon

gains energy (speed) by stealing Earth's spin energy and uses it to

gradually expand its orbit, moving away from the earth.

As the earth spins, the gravity of the moon pulls water, air, and

land up towards itself causing a bulge called the tide. Earth's

spin then pulls this tidal bulge ahead of the moon. So the tide

moves slightly faster than the moon and just east of it. Then the

increased gravity effect from the bulge racing ahead of the moon,

gradually, almost imperceptibly, pulls the moon forward in its own

orbit even faster. This also slows Earth's spin but at a much less

perceptible rate.

The end result is a kind of Earth-sponsored sling shot effect. The

moon has distanced itself (receded) from Earth in this way, by

hundreds of thousands of kilometers, in its long association with


It is calculated that the current orbit of the moon increases in

diameter by about 3.8 cm (about 1 1/2 inches) every year as a

result of this effect. Over millions and billions of years this has

added up.

The moon is currently moving away from the earth about 3 to 4 times

faster than in the past. The spacing of the major oceans and

continents allows for a more powerful effect than earlier. But it

will not continue this way indefinitely. As the continents continue

to drift, affecting the tides, the rate that the moon recedes will

also change.

Were the earth to stop spinning, this sling shot effect would stop

and the moon's orbit would gradually decay. So, the moon might come

crashing down upon the earth someday. Although, that day would

certainly be billions of years away.

In fact, current calculations are that the moon would finally stop

moving away from earth approximately 15 billion years from now,

with collision expected in more than 30 billion years. However, the

sun is expected to develop into a red giant and incinerate both the

earth and the moon long before then: 6 or 7 billion years from


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