# How do they figure the distance between celestial bodies?

## A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD

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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

SDStaff Chronos,Straight Dope Science Advisory Board replies:

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, we need to measure at least one distance. Usually, this is done by sending a radar pulse to either Mars or Venus, when it’s at closest approach to the Earth. Since we now know the difference |AMars or Venus – AEarth|, and we already knew the ratio PMars or Venus/PEarth, the rest is child’s play.