PHYS 3154 Observational Astrophysics J. H. Simonetti jsimonetti@vt.edu

Dates and Standard Coordinate Epochs

The current standard epoch of equatorial coordinates is J2000.0 which refers to the RA and Dec of an object on 2000 Jan 1 12UT also called 2000 Jan 1.5 (= noon time UT on January 1, 2000).

This is a Julian epoch, denoted by the "J". It is a Julian epoch since it is 100 x 365.25 days since the standard epoch J1900.0 = 1900 Jan 0 12UT = JD2415020.0 = 12 hours after 1900 Jan 0 UT = 12 hours after 0 UT between Dec 30 and Dec 31 1899 (!).

Digression: This counting from midnight between Dec 30 and Dec 31 is used since it means that after 1 day has elapsed you find yourself at the start of Jan *1* 0UT --- so the Jan 1 date is what everyone commonly knows as labeling the first day of the month of January: all events on that day occur on Jan 1 at some UT>0 (Duffett-Smith, Practical Astronomy with Your Calculator, p. 6).

Thus, RA and Dec J2000.0 are the apparent RA and Dec on 2000 Jan 1 12UT = JD2451545.0.

By contrast, RA and Dec B1950.0 (the old system) refer to apparent RA and Dec on 1950 Jan 0.923 = JD2433282.423 (Astronomical Almanac, Section B, Julian Date) = 1949 Dec 31 22.09 UT (http://www.xylem.f2s.com/kepler/b1950.html). The "B" in B1950 refers to a Besselian epoch.

Footnote: The Besselian system of time measured time in (tropical) years, and specifies a moment by giving the year and decimal fraction of year. This system has two problems: first, the tropical year varies in length slowly, and second, the beginnings of years in this sytem do not correspond to Julian dates, convenient for dynamical astronomy. The Besselian system has been replaced by a system whereby 100 years is exactly 32525 days (1 Julian century) and in which 1900.0 corresponds to 1900 Jan 0.5 (Explanatory Supplement to the Astronomical Alamanac, p. 8).

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