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Hour angle
 
 
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In astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
, the hour angle is one of the coordinates used in the equatorial coordinate system
Equatorial coordinate system

The equatorial coordinate system is probably the most widely used celestial coordinate system, whose equatorial coordinates are:* declination ...
 for describing the position of a point on the celestial sphere
Celestial sphere

In astronomy and navigation, the celestial sphere is an imagination rotation sphere of "gigantic radius", concentric spheres and coaxial with the Earth....
. The hour angle of a point is the angle between the half plane determined by the Earth axis and the zenith
Zenith

In broad terms, the zenith is the direction pointing directly above a particular location . Since the concept of being above is itself somewhat vague, scientists define the zenith in more rigorous terms....
 (half of the meridian plane) and the half plane determined by the Earth axis and the given point. The angle is taken with minus sign if the point is eastward of the meridian plane and with the plus sign if the point is westward of the meridian plane.

The hour angle is usually expressed in time units, with 24 hours corresponding to 360 degrees.

The hour angle must be paired with the declination
Declination

In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle....
 in order to fully specify the position of a point on the celestial sphere as seen by the observer at a given time.

>

Thus, the object's hour angle indicates how much sidereal time
Sidereal time

Sidereal time is a measure of the position of the Earth in its rotation around its axis, or time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical to, the motion of stars....
 has passed since the object was on the local meridian
Meridian (astronomy)

This article is about the astronomical concept. For other uses of the word, see meridian .In the sky, a meridian is an imaginary great circle on the celestial sphere....
.






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In astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
, the hour angle is one of the coordinates used in the equatorial coordinate system
Equatorial coordinate system

The equatorial coordinate system is probably the most widely used celestial coordinate system, whose equatorial coordinates are:* declination ...
 for describing the position of a point on the celestial sphere
Celestial sphere

In astronomy and navigation, the celestial sphere is an imagination rotation sphere of "gigantic radius", concentric spheres and coaxial with the Earth....
. The hour angle of a point is the angle between the half plane determined by the Earth axis and the zenith
Zenith

In broad terms, the zenith is the direction pointing directly above a particular location . Since the concept of being above is itself somewhat vague, scientists define the zenith in more rigorous terms....
 (half of the meridian plane) and the half plane determined by the Earth axis and the given point. The angle is taken with minus sign if the point is eastward of the meridian plane and with the plus sign if the point is westward of the meridian plane.

The hour angle is usually expressed in time units, with 24 hours corresponding to 360 degrees.

The hour angle must be paired with the declination
Declination

In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle....
 in order to fully specify the position of a point on the celestial sphere as seen by the observer at a given time.

Relation with the right ascension


The hour angle (HA) of an object is equal to the difference between the current local sidereal time
Sidereal time

Sidereal time is a measure of the position of the Earth in its rotation around its axis, or time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical to, the motion of stars....
 (LST) and the right ascension
Right ascension

Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system....
  of that object:

HAobject = LST - object


Thus, the object's hour angle indicates how much sidereal time
Sidereal time

Sidereal time is a measure of the position of the Earth in its rotation around its axis, or time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical to, the motion of stars....
 has passed since the object was on the local meridian
Meridian (astronomy)

This article is about the astronomical concept. For other uses of the word, see meridian .In the sky, a meridian is an imaginary great circle on the celestial sphere....
. It is also the angular distance between the object and the meridian
Meridian (astronomy)

This article is about the astronomical concept. For other uses of the word, see meridian .In the sky, a meridian is an imaginary great circle on the celestial sphere....
, measured in hours (1 hour = 15 degrees). For example, if an object has an hour angle of 2.5 hours, it transited across the local meridian 2.5 sidereal hours ago (i.e., hours measured using sidereal time), and is currently 37.5 degrees west of the meridian. Negative hour angles indicate the time until the next transit across the local meridian. Of course, an hour angle of zero means the object is currently on the local meridian.

Solar hour angle


The hour angle (h or ha) of a point on the earth's surface is the angle through which the earth would turn to bring the meridian of the point directly under the sun. The earth is rotating, so this angular displacement represents time.

So in observing the sun from earth, the solar hour angle is an expression of time, expressed in angular measurement, most usually degrees, from the solar noon.

At solar noon, at that observer's longitude on earth, the hour angle is 0.000 degrees with the time before solar noon expressed as negative degrees, and the local time after solar noon expressed as positive degrees.

The cosine of the hour angle (cosh) becomes an easy computation tool in determining the cosine term for the computation of the angle of the sun's altitude (or the complementary zenith angle) at any time during the day. At solar noon, h=0.000 so of course, cosh=1, and before and after solar noon the cos(± h) term = the same value for morning (negative hour angle) or afternoon (positive hour angle), i.e. the sun is at the same altitude in the sky at both 11:00AM and 1:00PM solar time, etc.