How Do We Tell Distance By Radar?
For relatively nearby things, like airplanes, we can use radar waves to measure how far away they are. Radar waves are invisible waves that are created and detected by electronic equipment. They are like the radio waves that your radio at home receives. You cannot hear radio waves directly; but your radio receiver can change them into the sounds that you enjoy listening to.
Let’s make a model of radar waves to see how they can be used to tell how far away something is. Remember, real radar waves are invisible, and make no sound at all. First, let’s just make a wave, like at a football game.
Raise your arms after the person next to you raises theirs. When the wave gets to the end of the row, the last person must say “Bounce!” and the wave goes back the other way. When it gets back to the starting point, the first person says “Stop!” Let’s try it once for practice.
Designate a starting person. If students are sitting in several rows, designate the end of each row as a “Bounce!” point. If they are sitting in a circle, have the person next to the starting person “bounce” the wave back so it goes around the circle in the opposite direction back to the starting person.
Now we will time how long it takes the wave to go the length of the whole row and back again. The starting person will help me be a timer by saying “One thousand one, one thousand two,” and so forth, to count approximate seconds.
(Optional: use audiotape of seconds being counted, or ask if anyone in the group has a timer function watch and would be willing to act as timer.)
Ready? Set. Go!
Start the wave and the timing simultaneously. If there is more than one row and the rows are different lengths, ask the students if they noticed any connection between how long the row is and how long it took the wave to return to the starting person.
Can you predict how long it will take for the wave to reach the end person if we cut the distance in half?
Appoint a new endpoint person somewhere about midway down the line, ask that person to “bounce” the wave. Start the wave and time it.
Air traffic controllers at airports use radar systems to measure distances to airplanes. Radio waves are sent out and they bounce off airplanes. The amount of time the wave takes to return tells the controller how far away the airplane is. The longer the time the wave takes to return, the farther away the airplane is.
Radar at airports measure distances to airplanes that may be a few dozen or at most a few hundred kilometers away. Astronomical things are much farther.
Distances to the Moon, Sun, and planets in the solar system were measured more than 300 years ago using a method we’ll learn about later called parallax. But today, timing waves sent to and from spacecraft, such as Pioneers, Voyagers, and a host of others, is one of the most accurate methods of determining distances within the solar system.
The method is similar to estimating the distance to a lightning strike by counting the number of seconds it takes for the sound of the thunder to reach you after you see the flash of the lightning. (The light travels so fast there is no perceptible delay.) Knowing the speed of sound waves, about a third of a kilometer per second, you can tell how far away the lightning was. (Multiply the number of seconds delay by 3 to get the approximate distance in kilometers.)