Equation of the equinoxes
Cross-traditionAstronomical factEditorial synthesis
Definition
The small difference between apparent and mean sidereal time, caused by nutation. It is reported in arcseconds and is the reason a chart states which of the two sidereal-time values it used.
Also called: Equation of the equinox
In more depth
Mean sidereal time is measured against a smoothly moving reference direction; apparent sidereal time is measured against the true equinox of the moment, which nutation shifts by a small periodic amount. The equation of the equinoxes is the difference between them, and it is quoted in arcseconds or in the equivalent fraction of a second of time.[1]
The quantity is tiny, but it is exactly the kind of convention that quietly separates two otherwise identical charts. Astro Gazer reports it, and reports both mean and apparent sidereal time, so that the choice is on the page rather than buried in the software.[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