We’ve been musing about the Moon for the last couple of weeks. The Moon won’t be in the evening sky this week, but there is something that you can do that will help your understanding when the Moon reappears in the evening sky next week. Today, we know the Earth’s spin gives us night and day as we rotate toward and away from the Sun. Ancients thought the sky revolved around the Earth as the Moon really does. What about the Moon? Does it rotate too?
Have you ever noticed that every time you look at a full Moon it looks the same? Even as recent as the Renaissance, astronomers thought the Moon was a mirror and reflected features on the Earth. They called the dark patches mare, the Latin word for seas. So, how can the features on the Moon always be the same and the Moon be rotating? Let’s do an experiment and find out.
You’ll need a partner whose head will represent the Earth and a Styrofoam ball on a stick to be the Moon. With a sharpie, draw a face on the ball. Now, have your partner watch the ball as you walk in a circle around them. Keep the face on the ball pointed at your partner, the Earth. OK, here’s the proof that the Moon rotates. Now, go around again, this time pointing your outstretched arm in the direction of your partner. Notice where you arm points as you move around. You have swept out a full circle. So has the ball representing the Moon!
When any moon orbits close to a more massive planet, it will also rotate in the same period as its orbit, and it will always keep the same face toward the planet. This seems like quite a coincidence, but it’s not. Most of the major moons of our solar system do this. Even some planets around other stars are known do this if they’re close to their star.
Astronomers have a name for this common tendency. It’s called “synchronous rotation” or “tidal lock”. Just to give it a name doesn’t explain how it happens though. Is there a reason why the spin and orbit rates come to the same period? Yes, gravity!
The closer two objects are, the greater the gravitational attraction they have. Consider the Moon. The strength of Earth’s gravity is strongest on the side of Moon closest to the Earth, and weakest on the side farthest from the Earth. The difference in these forces stretch the Moon and create tidal bulges on the Moon, making it slightly egg shaped. It is thought that the Moon rotated much faster when it formed than it does today. The tidal friction created by the stretching and squeezing of Moon slowly sapped away the Moon’s rotational energy, causing the Moon’s rate of rotation to slow down until its spin period was the same as its orbital period. At this point tidal friction ceased, and the rate of rotation was frozen. The Moon has been locked in synchronous rotation with one side facing the Earth ever since. Now you know.
For a fun animation on this topic watch https://www.youtube.com/watch?v=6jUpX7J7ySo. For questions or comments: James Hill, Mississippi NASA/JPL Solar System Ambassador. jhill6333@gmail.com
