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Did I do something wrong?

  1. Sep 9, 2005 #1
    The problem is -

    Consider the following accept constants:

    Radius of the moon = 1.74 x 10^6 m
    Average moon-earth distance = 3.84 x 10^8 m

    a) What is the apparent angle the diameter of the moon subtends, as seen from the earth? Answer in units of degrees.

    I doubled the radius of the moon to get the diameter, and then divided the result by the moon-earth distance. Then, I took the inverse tangent of the result.

    I get 51.92 degrees. My college website is telling me this is wrong. Do you get a different answer?
  2. jcsd
  3. Sep 9, 2005 #2


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    tan(51.92°) = 1.276, so it would appear there is an error in the computation.

    The ratio of 2*Lunar Diameter to Earth-Moon distance is ~0.009, so the tan-1 should be small - less than 1°.
  4. Sep 9, 2005 #3


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    Did you check that your calculator was set to the appropriate degrees/radians setting?
  5. Sep 9, 2005 #4


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    Why did you doubled the lunar radius before taking the inverse tangeant? The tan thing works on rectangle-triangles only. I say find the angle subtented by half of the moon, and then multiply that angle by 2 to get the total angle subtented by the diameter.
  6. Sep 9, 2005 #5
    Wouldn't you get the same answer?

    Well, the angle subtended by half the moon is 25.96. So 25.96*2 = 51.92
  7. Sep 9, 2005 #6


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    I get 0.2596; 100 times less.
  8. Sep 9, 2005 #7
    I thought 0.2596 needs to be converted into a percent? Is that the only mistake I made? :cry:

    Yeah I'm an idiot. Now that I actually picture how insane of a degree 51.92 would be, it's clear as day. Thanks for the help!
    Last edited: Sep 9, 2005
  9. Sep 10, 2005 #8


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    Even you have made a mistake by changing it into percentage, but I don't understand why you have to double the radius of the moon. Isn't it
    arc tan(radius of the moon/moon-earth distance) is the answer? Thank you.
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