Why can I see the whole moon during various non-full-moon phases?

  • Here's an image that describes the phenomenon I'm asking about. The very thin sliver to the left is of course the surface of the moon as lighted by the sun. But the rest of the moon is also faintly lighted -- by what?

    I'm guessing that some amount of "earthshine" -- sunlight reflected from the earth -- is the culprit here, but I've found no authoritative confirmation online. It would be great to have a link to a source that explains what's really going on.

    enter image description here

    "I've found no authoritative confirmation online". Pity indeed. Wikipedia even lacks the "citation needed", though Leonarda da Vinci is mentioned; tt'll be interesting to find that reference.

    @Evert Yes, but it is probably one of those very understandable "what word should I google"-cases. It is difficult today to find information about any what's-its-name topic. EDIT: Ah, damn, he does mention the keyword "earthshine", ouch...

    Perhaps NASA is a more credible source (with further links at the bottom): The Da Vinci Glow .

    Yes, it was basically a case of not knowing what to google. And yep, it turns out "earthshine" is a great keyword for getting google results on this (e.g., "moon illuminated by earthshine" reveals much) Funny, I didn't think to use the term "earthshine" until writing this question here -- another case where merely asking the question is enough to point at the answer.

  • LocalFluff

    LocalFluff Correct answer

    8 years ago

    The dark parts of the Moon are partially illuminated by "Earthshine". That is sunlight which is reflected from Earth to the Moon. Just like the ground on Earth is lit up a bit by the Moon at night, so is the ground on the Moon lit up a bit by the Earth in the lunar night. And even more so because Earth is much larger in the lunar sky, than is the Moon in our sky. And the surface of Earth is actually more reflective per area unit than is the surface of the Moon.

    Furthermore, a crescent Moon means that the Sun, Earth, and Moon are nearly lined up, which means that Earth is full as seen from the surface of the Moon. The full Earth, if my rough calculations are correct, is about 40 times as bright as the full Moon.

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