Stargazers: Our two-faced moon: Does the moon rotate, or doesn't it?

May has arrived, and with it comes some beautiful evening weather to help us enjoy our night sky.

This week, the full moon occurs on Wednesday, May 10, and, as the full moon always does, it will rise above the eastern horizon as the sun sets behind the western horizon.

A few early Native American tribes knew the full moon of May as the Flower moon, since flowers can be abundant during this month, while some knew it as the Corn Planting moon, or even the Milk moon.

All are wonderful stories from history, but there's one fact about this moon that seems to puzzle just about everyone.

All sky watchers who have ever watched the moon cycle through the heavens from full moon to full moon have surely noticed that we only ever see one side of the moon.

This simple observation naturally begs the question: "Does the moon rotate, or doesn't it?"

Around the time of the full moon, this is one of the most common questions I hear.

It seems that everyone has their own answer and is positive they are correct. So let's try a practical demonstration to help clear up the matter.

In this experiment, you will represent the Earth, sitting or standing in the center of a room.

Get a friend to represent the moon.

This person will need to walk completely around you to simulate a lunar orbit. The walls, ceiling and floor will represent the distant stars.

Let's first make the moon orbit the Earth without spinning on its axis.

In other words, have your friend choose a point on a distant wall and face it constantly as he or she circles you. From your position at the center, what do you see of your friend during the entire orbit?

Then try the same thing with your friend twirling around as he or she circles you. What can you see with the rotating moon?

Hmm ... it seems that in both cases, a terrestrial observer would see lunar features change from week to week.

In other words, over time we should see different sides of the moon. But that's not at all what we see. So what's going on?

The secret is that in order to keep the same features aimed toward the Earth, the moon must spin on its axis at the same rate as it orbits our planet.

To demonstrate this, have your friend walk one quarter of the way around the orbit. In order to keep the same face toward you, he or she must rotate one quarter of the way around.

Another quarter of an orbit, he or she does another quarter of a rotation, and so on.

It seems that the question of the moon's rotation has two equally correct answers, depending upon your viewpoint.

From the central Earth, the answer is "no, the moon doesn't rotate," but I'll bet that your lunar friend, having spun themselves into dizziness, will disagree.

Now, why this occurs and why the moon's far side appears so differently ... well, those are questions for another day, so stay tuned!

Visit Dennis Mammana



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