A member of a class of unevolved or slightly evolved Population I disk stars with an effective temperature, degree of evolution, metallicity, and kinematic property not very different from those of th
e Sun.
atmosphere consists of those layers of the Sun that allow photons to freely exit to space. The regions are: the photosphere, chromosphere, transition region and corona
The total radiant energy received vertically from the Sun, per unit area per unit of time, at a position just outside the Earth’s atmosphere when the Earth is at its average distance from the Sun. Rad
iation at all wavelengths from all parts of the solar disk is included. Its value is approximately 2.00 cal cm-2 min-1 = 1.37 kW m-2 and varies slightly (by approximately 0.1 %) from day to day primarily in response to the apparent size of sunspots blocking emission from the photosphere.
The amount of solar radiation in all wavelengths received per unit of time per unit of area on a theoretical surface perpendicular to the Sun's rays and at Earth's mean distance from the Sun. Its mean
value is 1367.7 W m^-2 or 1.37 x 106 erg sec^-1 cm^-2. In other words, the solar constant is the mean solar irradiance on the outer atmosphere when the Sun and Earth are spaced at 1 astronomical unit.
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: The position angle between the geocentric north pole and the solar rotational north pole measured eastward from geocentric north. The range in P is plus or minus 26.31°.
Bo: Heliographic latitude of the central point of the solar disk; also called the B-angle. The range of B is plus or minus 7.23°, correcting for the tilt of the ecliptic with respect to the solar equatorial plane.
Example: If (P, B) = (-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 the west.
L: 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 degrees/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 1200 UTC and is calculated for the present day by assuming a uniform sidereal period of rotation of 25.38 days. Once P, B, and L 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 solar surface.
Longitude. The angular distance from a standard meridian (0 degrees heliographic longitude), measured from east to west (0 to 360 degrees) 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 0000 UT) by using the synodic rate of solar rotation (27.2753 days, 13.2 degrees/day).