Suppose a person, let's say Neil, is on moon and Earth is experiencing a total solar eclipse so how should Neil perceive the Earth? This is also a question from KVPY (5/11/2017) question no. 22. as seen below

question on eclipse from KVPY 2017, document from Career Point

Shouldn't it be a small circle as compared to Earth? As size of moon is smaller than Earth and due to diffraction the spot will be smaller than the size of moon so why the answer is given as [B] as it should be [C]? Am I incorrect somewhere or the answer is incorrect?

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    $\begingroup$ Yeah, (C) should be the answer. See e.g. these photos from ISS. From the Moon the spot will take even smaller part of the Earth disk. $\endgroup$
    – Ruslan
    Commented Sep 15, 2021 at 16:03
  • $\begingroup$ In addition, here's a composite video from two satellites (one at the Earth-Sun L1 point, one geosynchronous) showing the path of the spot on the Earth's disc. $\endgroup$ Commented Sep 15, 2021 at 16:08
  • $\begingroup$ It is not diffraction that (principally) makes the spot much smaller than the moon. It is that the sun is an extended source, so for any observer there are multiple lines of sight (in different directions) leading to the sun. The darkest part of the spot is where the moon blocks all these rays (the umbra, which sees a total eclipse). Everywhere else the moon blocks only some rays (the penumbra, which sees a partial eclipse) or none (no apparent eclipse). Diffraction produces only minimal corrections on top of the geometric optics. $\endgroup$
    – HTNW
    Commented Sep 15, 2021 at 17:23
  • $\begingroup$ @HTNW Ok but doesn't diffraction make a minute difference? $\endgroup$
    – Gamin8ing
    Commented Sep 16, 2021 at 13:13
  • $\begingroup$ @Gamin8ing Of course: I say "Diffraction produces only minimal corrections on top of the geometric optics." But if it were just diffraction (i.e. if the sun were a really bright point source at infinity) the shadow would be moon-sized ± a few inches, maybe. The reduction in spot size that you actually notice is 99.9...% not due to diffraction. $\endgroup$
    – HTNW
    Commented Sep 16, 2021 at 13:54

1 Answer 1


enter image description here [C] is correct As moon's size is small compared to earth, moon will cast a small circular shadow on earth. !!You are correct!!

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    $\begingroup$ Hmmm! I was right! $\endgroup$
    – Gamin8ing
    Commented Sep 16, 2021 at 13:05

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