Most of us have mistaken satellites for stars at least once. It’s not hard to see why: both are bright pinpoints of light in the night sky, and both are stationary—or so it seems. While the stars above are fixed in their locations (without getting too Neil DeGrasse Tyson about it), satellites are constantly moving through the sky, generally at several thousand miles per hour. So why do they look still enough to be stars?
This handy dandy Demo Science science demo will help you explain this lil’ ol’ mystery to your students.
For this experiment, you’ll need a length of rope about ten feet long, a student to act as your assistant (do NOT choose Kevin), and a large, open space with some sort of stationary object in it. This last bit means you’ll likely be heading out of doors, so make sure all your students are properly attired before leaving the classroom. Nobody’s getting pneumonia on your watch!
In your open area, designate a stationary object of your choosing to stand in for Earth. Could be a tree, could be a park bench, could be a parked car. Doesn’t matter, really, as long as it’s not going anywhere for a while—for simplicity’s sake, we’ll just say it’s a tree from here on out.
Give your helper one end of the rope and have them stand with their non-rope holding hand on the tree. This student will be mimicking the earth’s daily rotation on a much smaller scale. Hold the other end of the rope yourself and stand far enough away to make the rope taut. You will be playing the part of a satellite—and, in fact, you will technically actually be a satellite, albeit one that requires far less wafer bonding and encapsulation to protect your electrical systems against space radiation.
Then, both you and your student assistant should start walking in the same direction around the tree. You need to walk at a fast enough pace to both keep the rope tight and maintain the same position around the circumference of the tree as your student. Might wanna tell the kid to go relatively slow so you don’t have to sprint around the dang tree.
That is literally all there is to it.
And That’s Science How?
Unless they’re total dumb-dumbs, your students will have noticed how much faster you had to move to stay in line with your assistant. The difference in speed is necessary because of the significant differences in the sizes of the circles in which you and your helper were walking and the need to cover those distances in the same amount of time.
Most satellites orbit the earth at roughly 22,500 miles and therefore must go really, really fast to maintain their geosynchronous position. They go all the way around the circumferential distance of the earth every 24 hours while maintaining the same position above the planet at all times. Earth goes all the way around its own circumferential distance in 24 hours (the above-mentioned daily rotation), but has to cover far less distance than a satellite, as the earth obviously does not orbit itself. Because artificial satellites are always in the same place relative to Earth, they don’t appear to be moving at all.