There’s a beautiful conjunction between the Moon and the planet Venus early this evening. Venus is the “evening star” – the brightest object in the night sky after the Moon. The Moon is a thin crescent – the Sun illuminates only a sliver of the lunar hemisphere that faces Earth.

We can’t see it, but the Moon is moving farther from us – by about an inch and a half per year. It’s been moving away since it was born, when Earth was young. In fact, that shift was one of the clues that led to the leading theory of how the Moon was born.

In the chaotic conditions of the early solar system, Earth was walloped by a planet about the size of Mars. That blasted debris into orbit around Earth. Much of that material quickly coalesced to form one or more moons. Today’s Moon is the only survivor.

The collision caused Earth to spin much faster, so a day was much shorter than it is now. Gravitational interactions between Earth and Moon have slowed us down. But they’ve also caused the Moon to slide farther away. The process isn’t smooth – the Moon speeds up and slows down. And it won’t stay smooth in the future.

Given enough time, the Earth-Moon system would reach a point when the same hemisphere of Earth would always face the Moon, and the Moon would stop moving away. But that time may never come. It could be so far in the future that the Sun will have expired – perhaps destroying Earth and its slip-sliding Moon.

Script by Damond Benningfield