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The angle may be measured in degrees or in time, with 24 h = 360° exactly. In celestial navigation, the convention is to measure in degrees westward from the prime meridian (Greenwich hour angle, GHA), from the local meridian (local hour angle, LHA) or from the first point of Aries (sidereal hour angle, SHA).
GMHA = Universal Time − Offset, the Greenwich Mean Hour Angle of the mean (fictitious) Sun. Here time and angle are quantities that are related by factors such as: 2 π radians = 360° = 1 day = 24 hours. The difference, EOT, is measurable since GHA is an angle that can be measured and Universal Time, UT
A culminating star on the observer's meridian is said to have a zero hour angle (0 h). One sidereal hour (approximately 0.9973 solar hours) later, Earth's rotation will carry the star to the west of the meridian, and its hour angle will be 1 h. When calculating topocentric phenomena, right ascension may be converted into hour angle as an ...
Time sight is a general method for determining longitude by celestial observations using a chronometer; these observations are reduced by solving the navigational triangle for meridian angle and require known values for altitude, latitude, and declination; the meridian angle is converted to local hour angle and compared with Greenwich hour angle.
The new definitions of Greenwich mean and apparent sidereal time (since 2003, see above) are: (,) = () (,) = () such that θ is the Earth Rotation Angle, E PREC is the accumulated precession, and E 0 is equation of the origins, which represents accumulated precession and nutation. [ 15 ]
The position of Z or X is described via its declination—the angular distance north or south of the equator (corresponding to its latitude)—and the hour angle—the angle between its meridian and the Greenwich meridian (corresponding to its longitude). If the observer knows the angles subtended by P, Z, and X, they can calculate their ...
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LHA = GHA + L is the local hour angle (+W/-E), Dec and GHA are the declination and Greenwich hour angle of the star observed. And Ho is the true or observed altitude, that is, the altitude measured with a sextant corrected for dip, refraction and parallax.