What is a control variable for my science experiment?

My independent variable is the distance from Earth and my dependent variable is the apparent diameter of celestial bodies. I have no clue what my control variable is and my teacher is not much help. In other words, HELP!!!!|||Your control variable must be a reference value that you can use to validate your measurements from Earth. That means that this value cannot change by very much, but since you are dealing with celestial bodies, nothing is ever a fixed distance from the Earth. But, with a very small built-in margin of error (which should be noted in your write-up of the experiment) you could use the angular size of the Moon or the Sun as your control variable. Hope this helps.