# The Physics Philes, lesson 63: Time for (P)Recess!

If you live in the U.S., today is Labor Day. Even though it seems like basically every store is still open and people are still working, I’m determined to take my day off of classes and do something fun. Luckily, Monday is physics day so everything worked out really well.

Last week we discussed the conservation of angular momentum. This week we’ll finish the topic of angular momentum by discussing precession.

Like basically every book I’ve read on the topic in the past few days, let’s start off by thinking about a spinning top. It doesn’t just spin in place, perfectly upright. It wobbles. Back in the day, when I used to play with tops (because I totally don’t play with tops anymore *cough*cough*), I thought that this wobble was just a manifestation of my inability to spin it correctly. Nope. It’s actually precession.

Precession is what we call that little wobble a top makes. The top wants to fall over because of, you know, gravity. But, thanks to the top’s spinning motion, it doesn’t fall. The torque keeps the top upright, but not perfectly. The tension between the desire to fall straight down and the desire to spin causes the axis of rotation of the top to trace a tiny circle.

This may seem a little esoteric, but it’s actually very important for astronomers. As I’m sure you know, the Earth rotates on an axis, much like a top (except at a 23.4 degree angle, but you already knew that, too). The Earth is not perfectly spherical; the planet bulges a bit at the equator. This means that the sun and moon can pull a bit more at the equator. This causes the axis of rotation to slowly change over time. As the Earth travels around the sun, the sun and moon both tug at different areas of the bulge. Maybe one of my paint drawings will help illuminate what I’m talking about.

Not to scale. Obviously.

OK, let’s pretend that’s the Earth. The dark black line is the axis of rotation, and the orange things that look a little like ears represent the bulge of the Earth at the equator. At certain times, the sun and moon will tug on one side of the Earth more than the others. Since the Earth already has quite a bit of angular momentum, the momentum that is added by the sun and moon are added. The magnitude remains the same, but the direction changes. Over time, the Earth’s axis to wobble a bit, or precess. (It’s worth noting that this isn’t a smooth sweep across the sky. There are actually little wobbles and shakes within the bigger precession wobble. These little shakes are called nutation.)

This is pretty weird when you think about it, whether you’re thinking about it using the top example or the Earth. But this is really just the rotational analog to uniform circular motion. Think about it this way: Say you’ve got a ball on a string. If the ball isn’t moving and you pull the string, the ball will move toward you. But, if the ball is moving and you pull the string in a direction perpendicular to the direction the ball is moving, the ball won’t approach you. It will move in a circle around you. The ball changes direction, but not speed. Kinda like what happens in precession.

Whelp, that’s it for angular momentum. What comes next? You’ll just have to stop by next week to find out.

Featured image credit: Marcy Keller