All I know is, is that measurement of sky objects from the earth are measured by arcseconds and arcminutes. 1 arcsecond is 1/3600th of a degree. Example: the moon, when it is full is appox. 30 arcminutes. This is somewhat different depending on your location on earth. Most astronomers use math to determine diameters by using apparent distants and arcseconds. I don't know if this helps but maybe it will point you in the right direction.