Equinox Epoch Explained

A number of coordinate systems have their longitudes defined by the "First Point of Aries" (FPA). This is the direction from the Earth to the sun at the time of the vernal equinox, when the sun seems to cross the Earth's equatorial plane from below. The FPA is not a fixed direction, but drifts by about 1.4 deg/century (50.26 arc-sec/year). In addition, there are tiny irregularities in this drift of the order of <1 arc-sec/year. The "Equinox Epoch" specifies which instantaneous FPA direction is used in specifying longitudes and whether the tiny irregularities have been smoothed or averaged out.

B1950.0 refers to the FPA at 22.152 UT of 31 December, 1949.

J2000.0 refers to a smoothed FPA at 12.00 UT on 1 January, 2000.

True-of-Date refers to the actual FPA at 00.00 UT on the date of interest.

Mean-of-Date refers to the smoothed FPA at 00.00 on the date of interest.

If you have any question/comments about the Helioweb service, contact:
Dr. Natalia Papitashvili, E-mail: Natalia.E.Papitashvili@nasa.gov Code 672, NASA/Goddard Spaceflight Center, Greenbelt, MD 20771