| epoch | has symbol T |  |
| has definition An orbital element representing the time of perihelion passage. | |
| has definition A point of time selected as a fixed reference. | |
| has definition An arbitrary fixed instant of time or date used as a chronological reference datum for calendars (see calendar), celestial reference systems, star catalogs, or orbital motions (see orbit). | |
| has definition A date and time that specifies the reference system to which celestial coordinates are referred. Prior to 1984 coordinates of star catalogs were commonly referred to the mean equator and equinox of the beginning of a Besselian year. Beginning with 1984 the Julian year has been used, as denoted by the prefix J, e.g., J2000.0. | |
| is an instance of orbital element | |
| is an instance of time | |
| orbital element | is required to define the size, shape, and orientation of an orbit in space | |
| see also mean element | |
| see also osculating element | |
| time | has unit time unit | |
| number | has value | |
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