The orbit of the Moon around the Earth isn't quite a circle. Since distance between the Earth and the Moon changes over time, the apparent diameter of the Moon also varies very slightly. With a digital camera, you can observe this by counting pixels. However, this variation is very small and not easily noticed.

Who discovered that the angular diameter of the Moon varies over time?

  • $\begingroup$ Anyone who kows the Moon's orbit around Earth is elliptical knows that unless the Moon expads and shirks monthly, its apparent diameter increases and decreases monthly. Anyone who noticed that the apaprent diameter of the Moon appears to decease and icnrease monthly would deduce that the Moon gets farther and closer to Earth monthly. I think that the angular diameter of the Moon msut have been measured in the first century of the use of telescopes in astronomy,a ndpossibly before the use of telescopes.. $\endgroup$ – M. A. Golding Oct 5 '20 at 18:39
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    $\begingroup$ "Very small" depends on how the diameter is being measured. A sextant with a resolution of 0.1 minutes (almost any metal sextant) would see yesterday's diameter as 30.2 minutes, today's 30.6, and tomorrow's 31.0. It might not see those exact numbers, but would see a change of 0.8 minutes. $\endgroup$ – stretch Oct 10 '20 at 0:43

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