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Is the moon orbiting the earth at the same speed that earth spins?

  1. Jun 2, 2013 #1
    I have always wondered why the moon doesn't look blurred to us if earth is spinning so fast, or are we spinning so fast that the image of the moon or everything else in the cosmos for that matter, looks clear? Like when a picture is taken of the earth it looks still, is it because the thing used to take the picture also in its orbit? would the spin only be visible if one were to get out of earth's orbit? Thanks.
     
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  3. Jun 2, 2013 #2

    mfb

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    One revolution per 24 hours is not "fast". You need a telescope with a good magnification to see this (apparent) motion in real time.

    Orbiting satellites have a velocity of ~8km/s (for low earth orbits), much more than the velocity due the rotation of earth (~500m/s at the equator). If you don't pay attention, images get blurry from there, even with a distance of ~400km.
     
  4. Jun 2, 2013 #3

    Janus

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    The Earth doesn't spin that fast. It takes 24 hrs for it to do one rotation. Look at the hour hand of a running clock; does it look blurred? Of course not, and it spins at twice the rate the Earth does.
     
  5. Jun 2, 2013 #4
    Thank you all very very much.
     
  6. Jun 2, 2013 #5
    To answer the other part of your question, the moon does not orbit at the same speed the earth spins, or else half the earth would never see the moon! Obviously, the moon orbits the earth in about 28 days, while the earth rotates in one day.

    It is tidally locked though, which is why we always see only one side of it, never the far-side.
     
  7. Jun 10, 2013 #6
    You can use 86,400 seconds as the length of the day, that times 28 days, the time it takes for one revolution of the moon around the Earth, is about 2,400,000 seconds. Using the distance to the moon as 225,000 miles (it varies but for this exercise, that's close enough), which is the radius of the more or less circular path the moon takes around the Earth, it's not circular but elliptical but lets call it circular for this discussion. So times 2 is the diameter, 450,000 miles, times pi makes that circle 1.4 million miles give or take. So divide 1,400,000 miles by 2,400,000 seconds and you find the moon traveling in its Earthy orbit going about 0.6 miles per second. Multiply that by 3600 and you get miles per hour, which is a bit over 2000 miles per hour. At the equator the Earth is spinning about 1000 miles per hour (it's about 25,000 miles around and it takes 24 hours so divide the two and you get close to 1000 miles per hour) So the moon is going in it's orbit twice as fast as the Earth spins on it's axis. Since the rate of the moon's spin is the same as it's orbital period of about 28 days, you always see only one side of the moon. (not completely correct since it does wobble a bit and you can see just a bit around the 'corner' of the moon)
     
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