Sidereal time
Cross-traditionAstronomical factEditorial synthesis
Definition
Time measured by the rotation of the Earth against the stars rather than against the Sun. It tells you which part of the sky is overhead at a given place and instant, which is what makes an Ascendant computable.
Also called: Local sidereal time · Greenwich sidereal time · LST
In more depth
A solar day is the time between successive noons; a sidereal day is the time the Earth takes to rotate once relative to the distant stars, and the two differ because the Earth also moves along its orbit. Sidereal time is the resulting timekeeping scale, and standard practice distinguishes mean from apparent sidereal time and Greenwich from local values.[1]
It matters to a chart because the angles depend on it. The Ascendant and Midheaven are defined by which point of the ecliptic is rising and culminating at the observer's location, and that requires knowing the Earth's rotational orientation at that instant, which is exactly what local sidereal time expresses. This is why an accurate birth time and place are needed for angles but not for the slower bodies.[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