Universal Time is the measure of Earth rotation that also serves as the astronomical basis of civil timekeeping. As the successor to Greenwich Mean Time (GMT), Universal Time intended to maintain uniform time as the angle between zero longitude and the mean sun—a fictitious point moving uniformly along the celestial equator that keeps pace with the Sun over the very long term. However, variability in the rotation rate of the Earth has caused Universal Time to diverge from the original geometric concept of mean solar time by approximately (1/365.2422) ΔT, where ΔT is the accumulated measure of nonuniform Earth rotation since 1900. After accounting for changes in the origin of the terrestrial and celestial reference systems since the end of the nineteenth century, simulated transits confirm that the Sun on average now crosses zero longitude at 12 h 00 m 00.2 s Universal Time, a result that is expected from theory.