Moon and Regulus

The first-quarter Moon huddles close to the heart of the lion tonight. Regulus, the brightest star of Leo, stands to the left of the Moon at nightfall, and even closer to the Moon at first light tomorrow. The star will stand to the right of the Moon tomorrow night.

Today, the Moon is about a quarter of a million miles from Earth. But that hasn’t always been the case. When the Moon was born, not long after Earth was, it was only a fraction of its current distance. And in the remote future, it’ll be much farther.

That’s thanks to the tides. The Moon’s gravity tugs at the oceans, creating “bulges” in the water. These bulges follow the Moon across the sky. As they slosh into the land, the bulges slow Earth’s rotation by a tiny amount.

To keep the books balanced on the overall motion of the Earth-Moon system, the Moon must move a little faster in its orbit. That causes it to move farther from Earth — by about an inch and a half per year.

Over tens of billions of years, things would reach a point where the same side of Earth would always face the Moon, just as the same side of the Moon always faces Earth. The Moon would stop moving away then, and the system would stay locked.

The evolution of the Sun complicates that picture, though. In a couple of billion years, the Sun will be so bright that it’ll boil away Earth’s oceans. No oceans, no tides — locking Earth and the Moon in a closer configuration.

Script by Damond Benningfield