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About space

  1. Oct 27, 2006 #1
    If a geo-synchoronise satelite(which has the same time period as that of the earth) comes in a straight line with the Sun. Does it catch the shadow on the earth? If not why?
  2. jcsd
  3. Oct 27, 2006 #2


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    Do a search on google for umbra and penumbra
  4. Oct 27, 2006 #3
    Basically, it is too small.

    Even the Moon, doesn't always cause a full eclipse - sometimes it is just slightly too far away (It's orbit isn't a perfect circle), and some sunlight gets round the edges.

    The Sun is about half a degree across - so the rays of light from each limb (side) as we look at it, are not parallel - they are converging (from our viewpoint) at an angle of about half a degree.

    So if you do the maths:-
    For a 2 metre wide satellite....
    Right angle triangle - quarter of a degree angle - 1 metre 'opposite' side (half the width of the satellite)
    Tangent of 0.25 degrees = one metre / adjacent side (distance behind satellite where light rays will converge

    0.00436 = 1 / adj
    adj = 1 / 0.00436
    adj = 229.3 metres

    So you'd need to be within 229 metres of the satellite to get a proper umbral shadow.

    Obviously - different satellites will be different sizes to the one in my example - so some will throw a shadow quite a bit more than 229m.
    However - satellites are AT LEAST tens of miles up, many are hundreds of miles above the surface - so even the largest ones won't throw a shadow anywhere near far enough for it to be seen on the ground.
    Last edited: Oct 27, 2006
  5. Oct 27, 2006 #4


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