Astronomy usually needs sidereal time. Mean solar time is a largely mathematical construct that has no direct relation to anything immediately observable. (Not

Astronomy usually needs sidereal time. Mean solar time is a largely mathematical construct that has no direct relation to anything immediately observable. (Notice the word 'mean' ... to _observe_ mean solar time you'd need to watch the sun for a whole year from the equator.) To the extent that astronomy applications want solar time at all, they probably wants it with more precision than you get from UTC.