HM Nautical Almanac Office
A new paper by F.R. Stephenson, L.V. Morrison and C.Y. Hohenkerk, Measurement of the Earths rotation: 720 BC to AD 2015 in Proceedings of the Royal Society A, presents new determinations of Earth Rotation (Delta T).
Today, Delta T is the difference between Terrestrial Time (TT) and Universal Time (UT1). It is a measure of the difference between a time scale based on the rotation of the Earth (UT1) and an idealised uniform timescale at the surface of the Earth (TT). TT is currently realised from International Atomic Time. Before atomic time, Ephemeris Time was used as the uniform timescale based on Newcombes Tables of the Sun. In order to predict the circumstances of an event on the surface of the Earth, such as the circumstances of a solar eclipse, a prediction of Delta T must be made for the instant of observation.
Definitions of technical terms may be found in the Glossary section of The Astronomical Almanac Online.