Most of the planets in our solar system trace essentially the same path around the sun: a circle. (Pluto, which is still and forever shall be a planet*, follows an elliptical orbit that sometimes crosses inside of Neptune’s). They all travel in the same direction in their orbits, as well. Some planets take much longer than others to complete one turn around our nearest star, however. Distance is, of course, the culprit, but if your students need a little more help envisioning why this is so—kids are pretty stupid, after all—this handy dandy Demo Science science demo will help you explain more better.
Set ‘Em Up, Knock ‘Em Down
This one requires just two pieces of equipment: a ruler and a yardstick. And gravity, I guess, but everyone has gravity all the time no matter what, so that doesn’t really count. Anyway, yeah, a ruler and a yardstick.
It’s also ridiculously simple. Hold the ruler and the yardstick up vertically, side by side, with their bottom edges resting on the floor or a table. Lean the linear measurement devices away from you slightly, to ensure they both fall in the same direction. Then, let ‘em go simultaneously and let gravity take over.
With a Shplit! and a Shplat! When they fall they’ll sound kinda like that! But what happened, and how does it relate to planetary orbits and such?
Making the Obvious More Obvious
The ruler and the yardstick were released simultaneously, and will hit the ground almost simultaneously, as well, but the ruler, being shorter, will in fact fall flat first. With less distance to fall, it makes for a much quicker trip.
Similarly, Mercury is far closer to the sun (“just” 35,980,000 miles) than Pluto (3.6 billion miles) and on a far shorter orbit, and, because of this, makes a much quicker trip around the sun. A year on Mercury takes only eighty-eight days; one Plutonian year takes 248 Earth years.
* Any student who says otherwise should immediately and automatically receive an F- in Science.