The earth completes one orbit around the sun in roughly a year. “Roughly” because it’s not exactly exact—hence, an extra day has to be built in every four years. (Three cheers for Leap Years!) That’s good for a speed of about 18.5 miles per second. Mercury, the planet closest to the sun, completes its orbit in a mere 88 Earth days (not Earth Days), while Pluto (still a planet, dang it) takes a leisurely 248 Earth years to make one trek around our star.
Given this info, it stands to reason that some planets must orbit faster than others. The length of their orbits varies wildly (Mercury is an average of 58.9 million kilometers from the sun, versus 5.9 billion kilometers for Pluto), which accounts for part of it, but if one maths it out, the planets’ speeds still don’t come close to matching up. What gives?
Science to the Rescue Explanation
With this easy peasy Demo Science science demo, you can demonstrate to your students how distance and gravity affect the speed of planets’ orbits. For this one, you’ll need a metal washer, a roughly 3-foot length of string, and a wide-open space in which to experiment.
First, tie one end of your string as tightly as possible to the washer. Then double knot it, just to be sure. Then triple knot it. Seriously.
Now, hold onto the string tightly by the opposite, non-washered end, and hold your swingin’ arm out in front of you. Swing the washer in a circle as slowly as you can whilst steel keeping the string taut the whole way ‘round its “orbit.” Have your students observe and take notes and whatnot.
Then, choke up a little bit—hold the string in the middle, and repeat the swingin’ and spinnin’. Again, notes, etc. Finally, hold the string about ten inches from the washer and repeat. What findings do your students, um, find?
Take It for A Spin
If they were paying attention (Kevin!), your students likely noticed that, as the length of the string decreased, the speed at which the washer had to spin to keep the string taut increased. That is: longer string equals slower spins, shorter string equals faster spins.
This directly correlates to the movement of the planets in their orbits. As a planet’s distance from the sun increases, the sun’s gravity doesn’t affect it as much and its orbital speed is lower. Pluto gets enough gravitational pull from the sun to keep it in orbit, but not enough to give it much oomph; the sun pulls hard enough on Mercury to keep it in a tight, fast orbit, but not quite enough to pull the planet into the sun and destroy it.