Does the Moon Rotate or Doesn't It?

By Dennis Mammana

April 11, 2019 4 min read

Week of April 14-20, 2019

Another month has passed, and another full moon shines in our sky this week. This one occurs on Friday, April 19, when we'll see our nearest cosmic neighbor rise in the eastern sky shortly after sunset.

The full moon of April is often known as the Pink Moon because, at this time of year, the grass pink or wild ground phlox is the most common flower around. Coastal North American tribes knew it as the Fish Moon, since the shad were now coming upstream to spawn. Other names include the Sprouting Grass Moon and the Egg Moon.

All are wonderful stories from history, but there's one puzzle about that moon that seems to have many folks confused. Sky watchers who have watched the moon cycle through the heavens from full moon to full moon have surely noticed that the features we see never change. In other words, we always see the same side of the moon.

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

Having discussed this curiosity with stargazers for more than four decades, I've concluded that there are two schools of thought about this issue — yes and no — and each person is absolutely certain that their reasoning is correct. So, let's try a practical demonstration to help clear up the matter.

In this experiment, you will represent the Earth by 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 in a lunar "orbit." The walls, ceiling and floor of the room will represent the distant stars.

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

OK, now try the same thing with your friend twirling around as he or she circles you. Now, with this "rotating moon," what can you see from the central "Earth"?

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 the Earth's gravitation has locked the moon into a situation where it spins on its axis at the same rate that it orbits our planet, and we, as a result, only see one side of it.

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, another quarter of a rotation — and so on.

It seems that the question "Does the moon rotate or doesn't it?" has two answers, depending on who you ask!

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!

 The moon can appear to be "two-faced," depending on what side of it you can see.
The moon can appear to be "two-faced," depending on what side of it you can see.

Visit Dennis Mammana at dennismammana.com. To read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

Like it? Share it!

  • 0

Stargazers
About Dennis Mammana
Read More | RSS | Subscribe

YOU MAY ALSO LIKE...