Timekeeping

Time kept by the real Sun, as a sundial shows it — noon is the moment the Sun crosses the meridian. Because the Earth's orbit is elliptical and tilted, the Sun runs fast or slow through the year, so clocks keep mean solar time instead, differing by up to about a quarter of an hour.

The time coordinate of the Solar System barycentre, used as the independent variable of the planetary ephemerides. It differs from Terrestrial Time only by small periodic terms arising from the Earth's motion around the Sun, never drifting by more than a couple of milliseconds.

Time kept by the stars rather than the Sun — it measures the Earth's rotation relative to the fixed sky. A sidereal day is about four minutes shorter than a solar day, because the Earth must turn a little further each day to face the Sun again as it moves along its orbit.

The idealised time on the Earth's surface used as the time axis for astronomical events and geocentric ephemerides. It runs at the same rate as International Atomic Time but is offset ahead of it by a fixed 32.184 seconds, for continuity with the older ephemeris time.

The extra second occasionally inserted into Coordinated Universal Time to keep it within 0.9 seconds of UT1, so that civil clocks stay in step with the turning of the Earth. As atomic time and Earth rotation drift apart, leap seconds bridge the gap — a practice now being reconsidered.

The form of Universal Time tied to the actual rotation of the Earth, measured from the positions of distant sources. Because the Earth's spin is slightly irregular and slowly slowing, UT1 drifts against the steady beat of atomic time.