Specifications for a location on the Solar surface. The location of a
specific feature on the Sun (for example, a sunspot) is complicated by the
fact that there is a tilt of 7.25 degrees between the ecliptic plane and the
Solar equatorial plane as well as a true wobble of the Solar rotational axis.
(Only twice a year are the Solar north pole and the celestial north pole
aligned.) Consequently, to specify a location on the Solar surface, three
coordinates (P, B, L) are necessary to define a grid. Daily values for the
coordinates in Coordinated Universal Time (UTC) are listed in The Astronomical
Almanac published annually by the U.S. Naval Observatory. The terms used to
refer to the coordinates are defined as follows:
- P-angle (or P): The position angle between the geocentric north pole and
the Solar rotational north pole measured eastward from geocentric north. The
range in P is +/- 26.3l °.
- Bo: Heliographic latitude of the central point of the Solar disk; also
called the B-angle. The range of Bo is +/- 7.23 °, correcting for the
tilt of the ecliptic with respect to the Solar equatorial plane. Example: If
(P, Bo) = (-26.21 °, -6.54 °), the heliographic latitude of the
central point on the Solar disk is -6.54 ° (the north rotational pole is
not visible), and the angle between the projection onto the disk of the
geocentric north pole and the Solar north rotational pole is 26.21 ° to
- Lo: Heliographic longitude of the central point of the Solar disk. The
longitude value is determined with reference to a system of fixed longitudes
rotating on the Sun at a rate of 13.2 °/day (the mean rate of rotation
observed from central meridian transits of sunspots). The standard meridian on
the Sun is defined to be the meridian that passed through the ascending node
of the Sun's equator on 1 January 1854 at 12:00 UTC and is calculated for the
present day by assuming a uniform sidereal period of rotation of 25.38 days.
Once P, Bo, and Lo are known, the latitude, central meridian distance, and
longitude of a specific Solar feature can be determined as follows:
- Latitude. The angular distance from the Solar equator, measured north or
south along the meridian.
- Central meridian distance (CMD). The angular distance in Solar longitude
measured from the central meridian. This position is relative to the view from
Earth and will change as the Sun rotates; therefore, this coordinate should
not be confused with heliographic positions that are fixed with respect to the
- Longitude. The angular distance from a standard meridian (0 °
heliographic longitude), measured from east to west (0 ° to 360 °)
along the Sun's equator. It is computed by combining CMD with the longitude of
the central meridian at the time of the observation, interpolating between
ephemeris values (for 00:00 UT) by using the synodic rate of Solar rotation
(27.2753 days, 13.2 °/day).