ΔT
In precise timekeeping, deltaT, or DT) is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of International Atomic Time (86,400 seconds). Formally, ΔT is the time difference between Universal Time (UT, defined by Earth's rotation) and Terrestrial Time (TT, independent of Earth's rotation). The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time.
ΔT (pronounced Delta T) is the time difference ΔT = TT − UT between Universal Time (UT, defined by Earth's rotation) and Terrestrial Time (TT, independent of Earth's rotation).
Universal Time (previously named Greenwich Mean Time, GMT) and Terrestrial Time (previously named Ephemeris Time, ET) are the two time scales essential for astronomical and astrological calculation. DeltaT is the difference between these two time scales.
UT is based on the Earth's rotation. Because the Earth rotation is slowing down due to tidal friction, a day or a second measured in UT does not always have the same length. Thousands of year ago, the Earth rotated a bit faster, and the day was a bit shorter. Some thousands of years from now, it will rotate slower, and the day will a bit longer.
A variable scale is not good for calculations. Mathematics and astronomy need a constant scale and a constant length of the day. This is why, when the changes in the Earth rotation period were recognized, astronomers introduced another time scale, called Ephemeris Time, and later Terrestrial time. This is the constant time scale measured by atomic clocks.
The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time. As well as this long-term drift in the length of the day there are short-term fluctuations in the length of day (Δτ) which are dealt with separately.
Since early 2017, the length of the day has happened to be very close to the conventional value, and ΔT has remained within half a second of 69 seconds. It is expected to rise again in the years after 2025.
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