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
Mean-of-Date refers to the smoothed FPA at 00.00 on the date of
If you have any question/comments about the Helioweb service, contact:
Dr. Natalia Papitashvili, E-mail:
Code 672, NASA/Goddard Spaceflight Center, Greenbelt, MD 20771