Universal Time (UT)

A clock set by the Earth's rotation

Universal Time (UT) is the time scale based on the rotation of the Earth. It is, in principle, mean solar time on the Greenwich meridian, counted from midnight: the successor of Greenwich Mean Time, adopted under that name by the International Astronomical Union in 1928. When an almanac, an eclipse prediction or this site gives an hour without further precision, it is UT.

Because the Earth is not a perfect clock, UT comes in several flavours. UT1 is the true rotation angle of the Earth, measured today with radio telescopes observing distant quasars (VLBI) and expressed as a time. UTC, Coordinated Universal Time, is the time broadcast by radio signals, GPS and the internet: an atomic time scale kept within 0.9 second of UT1 by adding leap seconds.

The time scales side by side

Civil time in each country is UTC plus a whole or half-hour offset: Paris is UTC+1 in winter and UTC+2 in summer, New York UTC−5 and UTC−4.

Leap seconds

The atomic second was defined in 1967 to match the length of the day in the 19th century. Since then the Earth has rotated slightly slower on average, mainly because tidal friction with the Moon brakes it, so a day lasts about 1 to 2 milliseconds more than 86,400 SI seconds. Left alone, an atomic clock would drift ahead of the Earth's rotation by roughly half a second to a second per year.

To keep the two together, a leap second, 23:59:60, is inserted at the end of June or December when needed. Twenty-seven have been added since 1972; the last one was on 31 December 2016. Since then the Earth has rotated a little faster, and no leap second has been needed. In 2022 the General Conference on Weights and Measures decided to let UT1 − UTC grow beyond 0.9 s by 2035, which in practice ends leap seconds.

UT versus Dynamical Time: ΔT

The planets follow the laws of gravitation, not the Earth's spin. Their positions are therefore computed in a uniform time scale, Terrestrial Time (TT), the modern form of Dynamical Time. The difference between the two is called ΔT:

ΔT = TT − UT1TT = UTC + 37 s + 32.184 s = UTC + 69.184 s (since 2017)

In 2026 ΔT is close to 69 seconds. It was about 33 s in 1960, close to zero around 1902, and grows to hours in antiquity. For ancient eclipses it is the main source of uncertainty: a ΔT error of one hour moves the path of totality by 15° of longitude.

Meeus's algorithms take TT as input for positions and UT for anything tied to the observer's horizon. A typical computation converts the observer's UT to TT by adding ΔT, computes the position of the body, then uses UT again to get the sidereal time and the local altitude and azimuth.

Worked example

Suppose you want the position of the Moon over Paris at 22:00 local summer time on 15 August 2026.

• Local time to UT: 22:00 − 2 h = 20:00 UTC, which is 20:00 UT1 within a second.
• Julian Day in UT: JD = 2461268.3333.
• Add ΔT ≈ 69 s = 0.0008 day: JDE = 2461268.3341 for the Moon's coordinates.
• Use JD (UT) for the Greenwich sidereal time, then the local hour angle, altitude and azimuth.

Mixing the two up for the Moon, which moves about its own diameter per hour, would shift it by about 0.6′ in 69 seconds, enough to spoil an occultation prediction but not a naked-eye view.

Universal Time on AstroAlgos

Every view on this site works in UT internally. The timeline and the date fields are converted to UT from your browser's time zone, then to a Julian Day; the Sun, Moon and planet theories use Dynamical Time, and the horizon views use sidereal time computed from UT.

Explore it on AstroAlgos

• Dynamical Time and ΔT — computed for any date
• Julian Day calculator
• Sidereal time — from UT to star time
• Night sky — the sky at your local time