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For the last few years…
The precisely defined time is
noon (UTC) on 2000 Jan 01
(ie. JD 2,451,545.0 – more of this later)
Thus one often sees “(J2000)” after equatorial coords
(and some other time-dependent quantities)
Example:
the (α,δ) of the Galactic Center
are α = 266.405100 O , δ = -28.936175 O (J2000)
… which as noted already is more often written as
17h 45m 37.224s, -28 O 56’ 10.23” (J2000)
Right Ascension
in time units
Declination
in angular units
Ian M. George PHYS 304 (2011 Fall) Meeting 04