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Yesterday evening, the moon from my place looked like this:

moon on september 2018, the 15th

Basically, instead of the crescent shape I am used to it looked like there was three tips: one at each extremity (as usual) and one in the middle between the two first ones. This picture shows the rough shape of the visible moon, with arrows pointing at all three tips:

annotated three tipped moon

What is the explanation of this?

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    $\begingroup$ As can be clearly seen, certain parts of the moon's illuminated surface are darker than others. So we have various degrees of light reflection on the illuminated side itself, plus a penumbral region, plus the completely dark side. (It's fifty shades of grey, basically). $\endgroup$
    – Lucian
    Sep 17, 2018 at 2:19

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Here's an image I captured from a Moon globe app on my iPad.

enter image description here

From your description, I think you may be referring to the southern rim of Mare Tranquillitatis, the Sea of Tranquility, perhaps extending a little into the Rimae Hypatia. This is an area which is lower than the surrounding mountains and craters, into which the shadow is encroaching.

Incidentally, Mare Tranquillitatis is the region where Neil Armstrong and Buzz Aldrin landed on the Apollo 11 mission!

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  • $\begingroup$ Seems like it was more Mare Serenitatis, but it makes sense. I wouldn't have thought that the mountains on the moon were high enough to create this kind of view $\endgroup$
    – Anne Aunyme
    Sep 17, 2018 at 14:25
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The lunar phase is based on the "age" of the moon since the last new moon. Any illumination up until a first quarter moon will look crescent-shaped, without any "nose". This crescent shape is a result of the positions of the Sun, Earth, and Moon in space. So, we can both observe and calculate the percent illumination and the shape of that illumination. In this particular case, the lunar terminator was running through a lunar mare, making it seem like the terminator was distorted.

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