ephemeris time
Noun: - (Astronomy) A uniform time scale defined by the orbital motion of the Earth and other planets, used as an independent variable in gravitational theories and ephemerides. It is a dynamical time scale based on the laws of motion, designed to be free of the irregularities found in solar time.
Ephemeris Time (ET) was the historical precursor to modern dynamical time scales like Terrestrial Time (TT). It is used in astronomical calculations and theories where a uniform time argument is required. - The precise calculation of the planet's position required the use of ephemeris time. - Ephemeris time was defined to provide a more uniform timescale than mean solar time.
- As a Standard: Ephemeris Time served as the standard for the definition of the second from 1960 to 1967, based on the tropical year 1900.
- The SI second was originally defined based on a fraction of the tropical year as measured in ephemeris time.
- Terrestrial Time (TT): The modern successor to Ephemeris Time, differing by a constant offset. It is a coordinate time scale for geocentric observations.
- Dynamical Time: A broader category of time scales, including Ephemeris Time and its successors, based on the motions of celestial bodies.
- Terrestrial Time (TT) (in its modern, redefined form, though not identical).
- Dynamical Time (as a general class).
While "Ephemeris Time" is a specific historical term, it is crucial to distinguish it from apparent solar time (based on the Sun's observed position) and mean solar time (averaged apparent time). Ephemeris Time is a theoretical, uniform scale derived from orbital dynamics.
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(astronomy) a measure of time defined by Earth's orbital motion; terrestrial time is mean solar time corrected for the irregularities of the Earth's motions
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