June is one of my favorite months and a perfect season to get out for some stargazing.
This week, however, the moon will be arcing across our early evening sky, so why not do some moongazing instead?
If you watch the moon this week, you’ll discover a few interesting things.
First, the moon seems to change its position from night to night. Early this week, you’ll see the moon in the southwestern sky at dusk, but each successive night around the same time — and that is key — it will appear farther to the east.
This shouldn’t surprise anyone; the moon orbits the Earth just about every month, so it needs to move across our sky to do that.
The second thing you’ll notice is that its shape — or phase — appears to change as it moves.
Remember, the moon is a hunk of rock a quarter of a million miles away, and it’s lit by sunlight. As the moon drifts in its orbit from night to night, the angle of sunlight falling on it changes.
Use binoculars to look more carefully at the moon this week, and you’ll find that you see the same surface features each night. The only conclusion one can make is that the moon must not rotate on its axis. Or does it?
Let’s try a practical demonstration to help clear up the mystery.
In this demonstration, you will represent the Earth, sitting or standing outdoors. Get a friend to represent the moon; this person will need to walk completely around you in a lunar “orbit.” Distant features on the landscape will represent the stars.
Let’s first have the “moon” orbit the Earth without rotating on its axis. In other words, have your friend choose a point on the distant horizon 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 entire orbit?
OK, now try the same thing with your friend spinning as they revolve around you. Now, with this “rotating moon,” what parts of your friend can you see from your position?
Well, that doesn’t help at all, does it? In both cases, a terrestrial observer would see lunar features change constantly.
In other words, over the next week, we should see different sides of the moon, but we don’t. So what’s going on?
The answer is that the moon does rotate, but it spins at the same rate as it orbits our planet.
To see this, try another demonstration. Have your friend walk one-quarter of the way around their orbit.
To keep the same face toward you, he or she must also rotate one-quarter of the way around. Another quarter of an orbit, another quarter of a rotation. And so on.
As you watch the moon each night this week, ponder this question: Does the moon rotate or doesn’t it? The answer is both yes and no, depending on your point of view.
From the central Earth, the answer is “no, the moon doesn’t rotate,” but from the perspective of your now-dizzy “lunar friend,” the answer is absolutely yes!
