The Moon is drifting away from us — at about an inch and a half per year. That’s not much, but it adds up. Billions of years ago, the Moon was much closer than it is now. And because of that, Earth’s days were much shorter.

According to a recent study, for example, two and a half billion years ago the Moon was only about 200,000 miles away — roughly 40,000 miles closer than it is now. And Earth’s day was only about 17 hours long.

The numbers are changing thanks to the tides. As the tides slosh back and forth against the land, they function as a brake, slowing down Earth’s rotation on its axis. Tides in the land contribute to the effect, as do tides in the lunar landscape. That makes the days longer. To keep the total motions of the Earth-Moon system in balance, the Moon has to move farther away.

But it’s not a smooth process — the rate of change varies. That’s caused by changes in the shape of Earth’s orbit, its tilt on its axis, and other factors. Right now, things are happening more quickly than they were a few billion years ago — causing the Moon to slide away faster than it did in the distant past.

The fat crescent Moon is in the south and southwest this evening. And it has a prominent companion — the giant planet Saturn. Saturn stands above the Moon at nightfall, and looks like a bright star. They’ll be a little farther apart tomorrow night, with Saturn to the right of the receding Moon.

Script by Damond Benningfield