The Julian Day (JD),
used in astronomical algorithms, is the number of
days since noon at Greenwich on 1 Jan 4713 BC.
The sidereal time is
measured by the rotation of the Earth, with
respect to the stars (rather than relative to the
Sun). Local
sidereal time is the right
ascension (RA, an equatorial coordinate) of a star
on the observers meridian. One sidereal day
corresponds to the time taken for the Earth to
rotate once with respect to the stars and lasts
approximately 23 h 56 min.
The sidereal time is a direct indication
of whether a celestial object of known right
ascension is observable at that instant. Applet of W.
Fendt:
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