We know that eclipse is when the Sun, the Earth and the Moon align in a straight line.
How do they do this align if their orbit planes aren't in the same angle?
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$\begingroup$ Have you tried drawing a diagram? $\endgroup$– G. SmithCommented Mar 16, 2020 at 22:34
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$\begingroup$ I don't know how to draw it in the best way $\endgroup$– Snack ExchangeCommented Mar 16, 2020 at 22:35
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1$\begingroup$ Planes intersect along a line. Normally we see a full Moon for the reason you cite, but just sometimes we align with the intersection plane and get a lunar eclipse. Swap the rôles of Moon and Earth and there's your answer. $\endgroup$– Selene RoutleyCommented Mar 16, 2020 at 22:36
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$\begingroup$ This may be helpful: en.wikipedia.org/wiki/Eclipse_season Also see en.wikipedia.org/wiki/Lunar_node $\endgroup$– PM 2RingCommented Mar 17, 2020 at 8:43
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2 Answers
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Planes intersect along a line. Normally we see a full Moon shining in Her fullness for exactly the reason you cite, but just sometimes the Moon passes through an intersection point exactly when occluded by the Earth and so we get a lunar eclipse. Swap the rôles of Moon and Earth and there's your answer.
answered Mar 16, 2020 at 22:39
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They are not in the same plane, but the planes intersect, and the relative orientation goes through 360 degrees every 365.2524 days.
The objection would be valid if the lunar plane were parallel to the earth's plane, but offset by more than one Earth radius, but that is not how it works.
By the way, the alignment is call "syzygy".
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1$\begingroup$ Why syzygy? It makes me think of David Bowie and Ziggy Stardust, but i'm letting slip my age .... $\endgroup$ Commented Mar 16, 2020 at 22:44
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$\begingroup$ of course, it is not really 360 degrees $\endgroup$ Commented Mar 16, 2020 at 23:06
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$\begingroup$ @SeleneRoutley I learned about the word studying isotropic tensors and Capelli's identities: mathworld.wolfram.com/Syzygy.html $\endgroup$– JEBCommented Mar 16, 2020 at 23:59