Question:

Hey! Star chasers I have one for you.?

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Since I am not a science major my question is this: The earth is rotating at approxmately a 1000 mph and the moon is revolving around earth once every twenty eight days while we're both revolving around the sun every 365 days, how come we never get to see the dark side of the moon from earth while the sun gets to see all 360 degrees of planet earth and the moon together? What's going on? Is there an invisable string between earth and the moon called gravity that keeps the moon looking straieght at earth without rotating away so we can't see it's dark side? Then, If that's true, how come the invisable string called gravity between earth and the sun dosen't stop us from spinning on our axis while we revolve around the sun?

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  1. As I understand it, the moon used to rotate more quickly, so that it did not keep the same face towards earth all the time. The gravitational pull of earth created tides in the lunar rocks, which caused the moon to bulge slightly on the side facing earth and the side facing away from earth. As the moon rotated, those bulges rotated with it. The bulge on the earth-facing side was pulled by gravity slightly more than the bulge on the far side. The net effect of this, over millions of years, was to slow down the moon's rotation.


  2. It does rotate, but it is in a syncrinicity with its orbit so the same side is always facing us.  Easier to understand if you use visual props.  Hold up 2 balls and move one around the other as if in orbit.  You will have to spin the orbit one around in order to keep the same point facing the inner one.

  3. And to further what Raymond said, Mercury is already tidally locked with the sun (as in: the same side of the planet Mercury faces the sun always).

  4. String?

    Yes there is.

    It is the tidal effect.  The result of the difference in gravity between the closest point and the furthest point of an object.  Or, if you want to sound scientific, the "gravitational gradient".

    It takes time for the tidal effect to slow things down.  The bigger (and massive) a body, the longer it takes to slow down its rotation.

    Jupiter is enormous.  It has stopped the relative rotation of all its major satellites so that they show Jupiter the same face.

    Earth has done the same to the Moon.  The Moon is trying to do the same to Earth and our day is getting longer by 1 second every 40,000 years (or so).

    Like I said: takes time.

    The Sun is working hard on Venus.  It is not yet "spin-locked" but then, the Sun has only had four or five billion years to work on it.

    Earth is more massive that Venus AND further away from the Sun.  The Sun will not have had time to do the same to us; it will go red giant well before that.  In fact the red giant phase (in another 5 billion years) will even come before we become spin-locked with the Moon.

    The tidal effect from the Moon is more than double that of the Sun, so that it would win the "spin-lock" contest, not the Sun.

  5. It's called tidal locking.  The moon rotates once in the same time that it takes to orbit once.  It does sort of rock back and forth a little.  This movement is called "libration".  It lets us see a little more than half of the Moon, though not all at once.

    So, the Moon's gravity raises the tides on Earth.  But the bulge of the tide rotates ahead of straight down, because the Earth rotates faster than the Moon's orbit.  So that bulge of water (and Earth) has it's own gravity that pulls on the Moon.  This turns out to slow the rotation of the Earth, transfering that energy to the orbit of the Moon.  The Moon is getting slightly farther from us.  It's something like an inch a year.  If this keeps up long enough, the Earth's rotation will match the Moon's orbit, which would be longer by then.  However, the Sun will run out of fuel (maybe 5 billion years from now) before this happens.

    Well, the Moon is thought to have formed when a Mars sized object struck the Earth soon after the Earth formed.  Much of the debris from the collision went into Earth orbit, and came together to form the Moon.  The Earth was rotating quite fast back then, and the Moon formed much closer in.  But the Moon material was already rotating close the the speed of it's orbit.  As the Moon hardened, the spot that had the biggest gravity soon pointed towards the Earth - which continues to enforce the tide lock.

    The far side is not always dark - for example the far side is fully lit up during New Moon, when the side we see is dark.

    That's enough of what you need to know to be a level one Moon expert - which in the trade is called a lunatic.

    The Eastern US gets to see a partial solar eclipse on August first.  There's still time to get together with a local astronomy club.  They'll be equipped and happy to show it to you safely - weather permitting.

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