How do they figure the distance between celestial bodies?
Dear Straight Dope: I’ve been wondering–what is the process that we use to measure the distance of objects in space? How do we really know that a planet is 1,200 light years away? — Mike, Rochester, NY You probably think there’s a simple answer to this question, Mike. What’s frightening is that this is it. But we figure you’re old enough to take it. There are a number of steps in the process, with the results of each step used to calibrate the next. To start with, we need to know the distances of things in the solar system. For this, we use something called Kepler’s Third Law. This states that for any object in the solar system, the orbital period P (in years) is related to the average radius of the orbit A by the formula P2 = A3. The period can be determined easily by going out at night and watching the planet or whatever move. Plugging in the P gives us the radius A in astronomical units, or AUs. An AU is the average distance from the earth to the sun. To figure the length of an AU,
