date of reference

Noun
  1. (astronomy) an arbitrarily fixed date that is the point in time relative to which information (as coordinates of a celestial body) is recorded (synset 115321930)
    referred to in: astronomy, uranology - the branch of physics that studies celestial bodies and the universe as a whole
    is a type of: date - the particular day, month, or year (usually according to the Gregorian calendar) that an event occurred
    same as: epoch

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