Is the moon's libration great enough that the Earth can be seen to "rise" and "set" anywhere?

This arises from someone's earlier question that got deleted while I was in the middle of answering it. The Earth in the moon's sky has a diameter of about 2 degrees of arc. Is the libration such that someone on the limb would see the whole disc rise above the horizon and sink below it?

3 Answers

  • ?
    Lv 6
    10 years ago
    Favorite Answer

    Interesting question.

    For those who don't know the word libration (which included me until about 5 minutes ago):

    "libration |līˈbrā sh ən| noun Astronomy

    an apparent or real oscillation of the moon, by which parts near the edge of the disc that are often not visible from the earth sometimes come into view."

    According to the articles below, the libration in latitude is 6.5 degrees and the libration in longitude is 7.6 degrees, so I would conclude that, yes, the earth would completely rise and set for some observers because, as you mentioned, the earth subtends an angle of roughly 2 degrees for moon-based observers.

  • 10 years ago

    Tom -

    I would certainly think so. I have not seen any examples taken from the lunar surface, but the common videos from earth show huge features dropping out of sight altogether off the limb. I think the total angular motion is about 7 degrees. So, I am fairly certain that it would be a real phenomenon on the lunar surface.

  • 10 years ago

    Yes it is. A photograph taken during Apollos moon mission showed the same thing.

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