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Measuring the total width of the Moon's umbra and penumbra at EM-L2, 64,700 km, how wide would the shadow be?

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  • $\begingroup$ Why would you keep your "sources" secret? $\endgroup$ – uhoh Apr 19 at 0:50
  • $\begingroup$ Have a look at this answer to the question What is the darkest orbit around Earth? I think your geometry is wrong. At EM-L2 it's the Earth that's behind the Moon, not the Sun. A solar eclipse at EM-L2 would be quite rare and transient. Also see Are there any (Lagrange) points in the Solar System in perpetual shade? $\endgroup$ – uhoh Apr 19 at 0:53
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    $\begingroup$ @ uhoh I didn't have much faith in my source that is why I did not cite it. I will look for it again. $\endgroup$ – Bob516 Apr 19 at 3:18
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    $\begingroup$ @uhoh You are correct, my geometry in wrong. $\endgroup$ – Bob516 Apr 19 at 3:25
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    $\begingroup$ When I figure it out I will. Assuming I do not mess up again. $\endgroup$ – Bob516 Apr 19 at 3:29
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The angle between the inner and outer edges of the penumbra is the same as the Sun's apparent angular diameter. At 1.0 au from the Sun, that's 32 arcmin or 0.0093 radian. At the distance from the Moon to EM-L2, this angle spans 0.0093 * 64700 km = 602 km. The Moon's diameter is 3474 km, so the umbra diameter would be 2872 km and the penumbra diameter would be 4076 km.

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  • $\begingroup$ Maybe you should say "At a distance equal to the EM-L2's distance"? At EM-L2 it's the Earth that's behind the Moon, not the Sun. EM-L2 is along the Earth-Moon line and has nothing to do with the Sun. A solar eclipse at EM-L2 would be quite rare and transient. $\endgroup$ – uhoh Apr 19 at 23:53
  • $\begingroup$ @uhoh When the Moon phase is full, EM-L2 sees the Sun in conjunction with the Earth and Moon. OP didn't ask about eclipses. $\endgroup$ – Mike G Apr 20 at 1:30
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    $\begingroup$ okay I'm calling EM-L2 being in the Moon's umbra an eclipse, I can't see any way that can't be a correct usage of the term. Let's do it this way: What fraction of the time is EM-L2 in darkness on average? $\endgroup$ – uhoh Apr 20 at 1:55

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