GMSTdeg15

GMSTdeg15 is a quantity used in astronomy and navigation that represents Greenwich Mean Sidereal Time expressed in degrees. Since sidereal time completes a 360-degree rotation in 24 hours, GMST in hours can be converted to degrees by multiplying by 15. Therefore, GMSTdeg15 = GMST_hours × 15. The underlying concept, GMST, is the angle between the Greenwich meridian and the vernal equinox as projected onto the celestial equator, measured in sidereal time. GMST increases at approximately the rate of Earth's rotation relative to the stars and is used to transform celestial coordinates to terrestrial coordinates and to compute Local Sidereal Time at a given location.

A common way to compute GMSTdeg15 uses Julian Date (JD) and centuries since J2000.0, T = (JD − 2451545.0)/36525. The Greenwich Mean Sidereal Time in degrees is given by

GMST_deg = 280.46061837 + 360.98564736629 × (JD − 2451545.0) + 0.000387933 × T^2 − T^3/38710000

The result is typically reduced modulo 360 to lie in the range [0, 360). GMSTdeg15 is simply this degree measure, often used directly in software and calculations where angular units are preferred over time units.

An equivalent, shorter approximation for a specific UT date expresses GMST in hours and then multiplies by 15 to obtain degrees. Note that different formulations may use UT, Julian centuries, or mean solar time conventions, but all converge to the same degree-based representation of Greenwich sidereal time.

See also: Greenwich Mean Sidereal Time, Local Sidereal Time, celestial coordinates.