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The cosine of the hour angle (cos(h)) is used to calculate the solar zenith angle. At solar noon, h = 0.000 so cos(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 Greenwich hour angle of the body observed; L H A = G H A + L o n {\displaystyle LHA=GHA+Lon} , the local hour angle of the body observed. First calculate the altitude of the celestial body H c {\displaystyle Hc} using the equation of circle of equal altitude :
More exactly, sidereal time is the angle, measured along the celestial equator, from the observer's meridian to the great circle that passes through the March equinox (the northern hemisphere's vernal equinox) and both celestial poles, and is usually expressed in hours, minutes, and seconds.
The solar azimuth angle is the azimuth (horizontal angle with respect to north) of the Sun's position. [ 1 ] [ 2 ] [ 3 ] This horizontal coordinate defines the Sun 's relative direction along the local horizon , whereas the solar zenith angle (or its complementary angle solar elevation ) defines the Sun's apparent altitude .
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.
If the observer knows the angles subtended by P, Z, and X, they can calculate their position on the globe. By measuring the angle of the celestial body in the sky, the observer can get the local hour angle (LHA) of X, which is the angle subtended at P between Z and X (the angle between the Z and X's meridians ) and calculate the longitude by ...
Alternatively to right ascension, hour angle (abbreviated HA or LHA, local hour angle), a left-handed system, measures the angular distance of an object westward along the celestial equator from the observer's meridian to the hour circle passing through the object. Unlike right ascension, hour angle is always increasing with the rotation of Earth.
To calculate the azimuth of the Sun or a star given its declination and hour angle at a specific location, modify the formula for a spherical Earth. Replace φ 2 with declination and longitude difference with hour angle, and change the sign (since the hour angle is positive westward instead of east). [citation needed]