Does the length/distance of a planet’s orbit around the sun, or a moon’s orbit around its planet, affect the speed with which it moves in that orbit? You bet your sweet bippy it does! After all, Mercury has by far the shortest/fastest orbit of any planet in our solar system, while Pluto (still a planet in my book, dang it) has the longest/slowest orbit.
The farther a planet is from the sun (or the further from the planet a moon is), the lesser the effect of the sun’s (or the planet’s) gravity on that orbiting celestial body. Here’s a nifty little experiment you can use to demonstrate how this outer space magic works.
Doing That String Swing Thing
For this demo, all you’ll need is a metal washer about the size of a quarter, a roughly three-foot length of string, and an open area where you won’t clobber anything with your science. Tie one end of the string through the whole in the washer, then mark the string at the middle and at about 10 inches to a foot from the washer. (I guess you need a permanent marker for this, too.)
Hold the loose end of the string in one hand, and extend your arm away from your body. Keeping your arm more or less steady, start swinging the washer in circles. The idea is to spin the washer as slowly as possible, while still keeping the string taut. You’ll probably have to start out swinging it pretty fast, just to get it going, but then you can slow down a bit—this will probably require a bit of practice, so you may want to try it out ahead of time.
After you’ve given your audience a good look at that, hold the string at your center mark and repeat the process. After you’ve given it a few good spins at that length, hold the string at the one foot-ish mark and repeat again. Remember, the key here is to spin the washer around as slowly as you can while keeping it spinning in a complete circle (around your hand).
What Have We Learned?
Any students who were paying attention will have noticed that, as the length of the string (orbit) decreased, the washer (planet) had to spin faster to keep the string tight as it rotated around your hand (the sun). The speed of the washer at the full string length will seem very, very slow compared to the shorter-length spins.
This science demo shows the relationship between planetary orbit distance and orbit speed. The greater the distance from the sun, the less its gravity pulls on planets. The lower the gravitational effect, the slower the orbit. Science FTW!