The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane. This conveniently divides the sky into the upper hemisphere that you can see, and the lower hemisphere that you can't (because the Earth is in the way). The pole of the upper hemisphere is called the zenith. The pole of the lower hemisphere is called the nadir. In astronomy, a celestial coordinate system is a coordinate system for mapping positions in the sky. ...
Horizon The horizon is the line that separates earth from sky. ...
The fundamental plane in a spherical coordinate system is a plane which divides the sphere into two hemispheres. ...
The term hemisphere is used in three different meanings: onehalf of the Earth (or other planetary or stellar body; see also New World and Old World) Eastern Hemisphere and Western Hemisphere. ...
This article is about an astronomy term. ...
In astronomy, the nadir is the, obviously invisible, point on the sky vertically downward, i. ...
The horizontal coordinates are:  altitude (Alt), that is the angle between the object and the observer's local horizon.
 azimuth (Az), that is the angle of the object around the horizon (measured from the North point, toward the East).
The horizontal coordinate system is sometimes also called the Alt/Az coordinate system. Altitude is the elevation of an object from a known level or datum, called zero level. ...
Azimuth is the horizontal component of a direction (compass direction), measured around the horizon from the North point, toward the East, i. ...
The horizontal coordinate system is fixed to the Earth, not the stars. Therefore, the altitude and azimuth of an object changes with time, as the object appears to drift across the sky. In addition, because the horizontal system is defined by your local horizon, the same object viewed from different locations on Earth at the same time will have different values of altitude and azimuth. Horizontal coordinates are very useful for determining the rise and set times of an object in the sky. When an object's altitude is 0°, it is:  rising (if its azimuth is less than 180°)
 setting (if its azimuth is greater than 180°)
and there are the following special cases:  on the Poles, objects on the celestial equator turn around the horizon
 on the equator, objects on the celestial poles stay at one point on the horizon
Transformation of coordinates
It is possible to pass from the equatorial coordinate system to the horizontal coordinate system. The equatorial coordinate system is probably the most widely used celestial coordinate system, whose equatorial coordinates are: declination () right ascension () or hour angle () It is the most closely related to the geographic coordinate system, because they use the same fundamental plane, and the same poles. ...
Let δ be the declination, H the hour angle, φ the observer's latitude. In astronomy declination (dec) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. ...
In astronomy, an objects hour angle (HA) is defined as the difference between the current local sidereal time (LST) and the right ascension () of the object: HAobject = LST  object Thus, the objects hour angle indicates how much sidereal time has passed since the object was on the local...
Latitude, denoted by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ...
The equations of the transformation are:
Use the inverse trigonometric functions to get the values of the coordinates. In mathematics, the trigonometric functions are functions of an angle, important when studying triangles and modeling periodic phenomena. ...
The position of the Sun There are several ways to compute the apparent position of the Sun in horizontal coordinates. Complete and accurate algorithms to obtain precise values can be found in Jean Meeus's book Astronomical Algorithms. Jean Meeus (born 1928) is a Belgian astronomer. ...
Instead a simple approximate algorithm is the following: Given: You have to compute: Datateknologerna vid Åbo Akademi r. ...
8:17 am, August 6, 1945, Japanese time. ...
Latitude, denoted by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ...
Map of Earth showing curved lines of longitude Longitude, sometimes denoted Î», describes the location of a place on Earth east or west of a northsouth line called the Prime Meridian. ...
Time zones are areas of the Earth that have adopted the same standard time, usually referred to as the local time. ...
 The Sun declination of the corresponding day of the year, which is given by the following formula:
In astronomy declination (dec) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. ...
where N is the number of days spent since January 1. January 1 is the first day of the calendar year in both the Julian and Gregorian calendars. ...
 The true hour angle that is the angle which the earth should rotate to take the observer's location directly under the sun.
 Let hh:mm be the time the observer reads on the clock.
 Merge the hours and the minutes in one variable T = hh + mm/60 measured in hours.
 hh:mm is the official time of the time zone, but it is different from the true local time of the observer's location. T has to be corrected adding the quantity + (Longitude/15  Time Zone), which is measured in hours and represents the difference of time between the true local time of the observer's location and the official time of the time zone.
 If it is summer and Daylight Saving Time is used, you have to subtract one hour in order to get Standard Time.
 The value of the Equation of Time in that day has to be added. Since T is measured in hours, the Equation of Time must be divided by 60 before being added.
 The hour angle can be now computed. In fact the angle which the earth should rotate to take the observer's location directly under the sun is given by the following expression: H = (12  T) * 15. Since T is measured in hours and the speed of rotation of the earth 15 degrees per hour, H is measured in degrees. If you need H measured in radians you just have to multiply by the factor 2π/360.
This article's initial version originated from 'Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml In astronomy, an objects hour angle (HA) is defined as the difference between the current local sidereal time (LST) and the right ascension () of the object: HAobject = LST  object Thus, the objects hour angle indicates how much sidereal time has passed since the object was on the local...
Daylight saving time (also called DST, or Summer Time) is the portion of the year in which a regions local time is advanced by (usually) one hour from its standard official time. ...
Universal Time (UT) is a timescale based on the rotation of the Earth. ...
During the course of the year, the time as read from a sundial can run ahead of clock time by as much as 16 min 33 s (around October 31–November 1) or fall behind by as much as 14 min 6 s (around February 11–12). ...
The horizontal coordinate system is a celestial coordinate system that uses the observers local horizon as the fundamental plane. ...
Screenshot of KStars showing the night sky from Hanover. ...
Tux, a cartoon penguin frequently featured sitting, is the official Linux mascot. ...
KDE (K Desktop Environment) is a free desktop environment and development platform built with Trolltechs Qt toolkit. ...
