minipost
The Earth and Moon are gravitationally locked with the Moon rotating around its axis in the same time it takes to travel around the Earth. The result is we only see one side of the Moon .. Well... sort of.. The Moon doesn't orbit the Earth. Instead both Earth and Moon orbit a common point known as a barycenter which is about 1,700 km below the surface of the Earth. The Moon's orbit is also elliptical which turns out to impact how much of the Moon's surface you can see.
The Moon moves slower when it's farthest away and faster at its closest. You see a bit more of the Eastern side when it's moving slower and more of the Western side when it's speeding. Added together we can see about 59% of the Moon's surface over the course of a month. (There's also a smaller effect due to parallax, but it's mostly the elliptical orbit.)
You learn this right away when you start looking at Moon, but seeing a visualization is dramatic. Andrew McCarthy took two million photos over nearly a full lunar month and put together a wonderful video of the effect.
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