Delta-T
Cross-traditionAstronomical factEditorial synthesis
Definition
The difference between the uniform time scale used for planetary calculations and time measured by the Earth's actual rotation. The Earth is not a perfect clock, so the gap has to be measured, not predicted.
Also called: ΔT · DUT1 · UT1
In more depth
Planetary motions are computed on a uniform dynamical time scale, but civil time ultimately follows the rotation of the Earth, which is slightly irregular and gradually slowing. Delta-T is the offset between the two. For past dates it is derived from observations, including historical eclipse records; for the future it can only be extrapolated.[1]
A related quantity, DUT1, is the small difference between UT1 (time by the Earth's rotation) and the UTC that clocks display. Both matter for the same reason: the angles of a chart depend on the Earth's true orientation at the birth instant, so the software has to know how far the rotating Earth had actually turned, not just what a wall clock said.[1]
References
Numbered citations in the text link to the records below; each record links to the full source registry.
- Sean E. Urban (editor); P. Kenneth Seidelmann (editor). Explanatory Supplement to the Astronomical Almanac. Sean E. Urban and P. Kenneth Seidelmann. University Science Books. 2013. Astronomical fact View source record