Published: 17 June, 2009

Here's why the U.S. flag was waving on the moon

Since there is no air on the moon, why is the United States flag waving around when you see footage of man on the moon?

This is an exciting question, because it highlights how hard it is to notice things that are always around us, something even the great Aristotle had difficulty with. We're used to thinking of air making flags wave. When the wind blows, it can flutter a flag. But the air is there even when the wind isn't blowing, and stationary air acts as a damper to a flag's motion.

Imagine waving your arm back and forth in a bunch of water. It takes a lot more work than waving your arm in air. You have to move the water out of the way as you move back and forth. You have to do similar, but less, work to move the air out of the way when waving. You usually don't notice because it's a smaller amount of work, and in daily life it's always there. You're used to it.

As you pointed out though, there's no air on the moon. That lack means something moving doesn't have to use energy to move air out of the way. The flag waves because the astronauts who planted the flag touch it and make it shake. It then continues to wave longer than we expect because there is no atmosphere to dampen the motion. It does eventually stop waving because the bending and unbending of the fabric in the flag also dissipates energy.

This question, as well as several other moon-mission related oddities, was investigated by the show MythBusters. A clip is currently online showing an experiment they conducted in a vacuum chamber (check out http://tinyurl.com/btb3sw). Like the folks in the show, keep asking good questions about the world(s) around you.