The "missing bit" when the moon isn't quite round is actually the shadow of the Earth. This happens during a lunar eclipse.

Here's why:

* Lunar Eclipse: A lunar eclipse occurs when the Earth passes directly between the Sun and the Moon. The Earth casts a shadow on the Moon, making it appear partially or completely dark.

* Earth's Shadow: The Earth's shadow has two parts: the umbra (the darkest part) and the penumbra (a lighter, less dark area).

* Missing Bit: The "missing bit" you see on the Moon during a lunar eclipse is actually the part of the Moon that's within the Earth's umbra. The part of the Moon in the penumbra appears less bright, but not completely dark.

So, the "missing bit" is not actually missing, it's simply covered by the Earth's shadow!