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Inastronomy andcelestial navigation, thehour angle is thedihedral angle between themeridian plane (containingEarth's axis and thezenith) and thehour circle (containing Earth's axis and a given point of interest).[1]
It may be given in degrees, time, or rotations depending on the application.The angle may be expressed as negative east of the meridian plane and positive west of the meridian plane, or as positive westward from 0° to 360°. The angle may be measured in degrees or in time, with 24h = 360° exactly.Incelestial navigation, the convention is to measure in degrees westward from theprime meridian (Greenwich hour angle,GHA), from the local meridian (local hour angle,LHA) or from thefirst point of Aries (sidereal hour angle,SHA).
The hour angle is paired with thedeclination to fully specify the location of a point on thecelestial sphere in theequatorial coordinate system.[2]
The local hour angle (LHA) of an object in the observer's sky isorwhere LHAobject is the local hour angle of the object, LST is thelocal sidereal time, is the object'sright ascension, GST isGreenwich sidereal time and is the observer'slongitude (positive east from theprime meridian).[3] These angles can be measured in time (24 hours to a circle) or in degrees (360 degrees to a circle)—one or the other, not both.
Negative hour angles (−180° < LHAobject < 0°) indicate the object is approaching the meridian, positive hour angles (0° < LHAobject < 180°) indicate the object is moving away from the meridian; an hour angle of zero means the object is on the meridian.
Right ascension is frequently given in sexagesimal hours-minutes-seconds format (HH:MM:SS) in astronomy, though may be given in decimal hours, sexagesimal degrees (DDD:MM:SS), or, decimal degrees.
Because the earth rotates 365.2564 times in a sidereal year whereas fixed stars appear to go around one time more, the hour angle of a fixed star increases by 366.2564/365.2564 (about 1.0027) per hour, or in other words it takes 59 minutes and 50.17 seconds for the hour angle to increase by one hour.
Observing the Sun from Earth, thesolar hour angle is an expression of time, expressed in angular measurement, usually degrees, fromsolar noon. At solar noon the hour angle is zero degrees, with the time before solar noon expressed as negative degrees, and the local time after solar noon expressed as positive degrees. For example, at 10:30 AM local apparent time the hour angle is −22.5° (15° per hour times 1.5 hours before noon).[4]
The solar hour angle increases on average by one hour per hour, but because of theequation of time this varies with time of year. In mid-September a solar day is about 22 seconds less than 24 hours, meaning that the solar hour angle increases by 1.00025 hours per hour, whereas in late December a solar day is about 28 seconds more than 24 hours, so the solar hour angle increases by 0.99968 hours per hour.
Thecosine of the hour angle (cos(h)) is used to calculate thesolar zenith angle. At solar noon,h = 0.000 socos(h) = 1, and before and after solar noon the cos(± h) term = the same value for morning (negative hour angle) or afternoon (positive hour angle), so that the Sun is at the same altitude in the sky at 11:00AM and 1:00PM solar time.[5]
The sidereal hour angle (SHA) of a body on the celestial sphere is its angular distance west of theMarch equinox generally measured in degrees. The SHA of a star varies by less than a minute of arc per year, due toprecession, while the SHA of a planet varies significantly from night to night. SHA is often used incelestial navigation and navigational astronomy, and values are published in nauticalalmanacs.[citation needed]
