Epoch
Cross-traditionAstronomical factEditorial synthesis
Definition
The reference moment a coordinate system is defined for. Because precession keeps moving the reference directions, a celestial coordinate is only unambiguous once you say which epoch it belongs to.
Also called: Equinox of date · Standard epoch · Epoch basis
In more depth
Celestial coordinates are measured from directions (the equinox and the equator) that themselves move. A coordinate system therefore has to be pinned to a moment. Positions may be given for a standard epoch shared across astronomy, or for the equinox and equator of the date of observation itself.[1]
Published ephemerides state the frame and epoch basis their coordinates are expressed in, and consumers of the data are expected to carry that label with the numbers. Astro Gazer works in coordinates of date and says so in the calculation details, so the epoch is never left for the reader to guess.[2]
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
- Ryan S. Park; William M. Folkner; James G. Williams; Dale H. Boggs. The JPL Planetary and Lunar Ephemerides DE440 and DE441. The Astronomical Journal 161 (3), article 105. 2021. Astronomical fact View source record