What is the difference between UT0, UT1 and GMT time?

Question

Every reference I find says that they are essentially the same, which we all know really means that they are not the same, but different only by a some small amount. They also like to say that UT1 and UTC are the same, but if you look into it you find that they differ by as much as 0.9 seconds. What are the specifics and exactness where applicable here?

Answer 1

UT0 represents uncorrected observations of the distant stars / quasars by single stations. Due to higher precision needed for long baseline astronomy UT0 is an outdated concept.

UT1 is a specific "flavor" of Universal Time, which is a measure of Earth's rotation relative to the mean sun, a fictitious "prime mover" upon which all our clocks are based. UT1 is related to sidereal time (Earth's rotation relative to the fixed background stars) by a rather long mathematical expression. Earth's rotation is not uniform though; it varies. UTC (Coordinated Universal Time) is a specific "flavor" of Universal Time intended to smooth out these variations by staying within 0.9 s of UT1.

GMT, Greenwich Mean Time, is a sometimes deprecated historic term equivalent to UT, but is no longer used in astronomical applications. It used still used in many civil applications though (and remains the legal time standard (outside the period of daylight-saving summer time) in the UK).

Pulled these answers off https://physics.stackexchange.com.