Horizontal frame of reference

The horizontal Frame of reference, or local system , is a celestial Frame of reference used in Astronomie attached to a terrestrial observer.

Its datum-line is the horizontal plan, perpendicular to the Verticale of the place.

An object is located in this system:

by the angle, located in a vertical plan, between the direction of this object and a horizontal direction: its Height H ; this angle, expressed in degrees, lies between 0° and 90° (zenith); negative values remain however possible at the time of an observation starting from a high place;
by the angle enters the projection of the direction of the object on the horizontal plane and the direction of the true north from this same point of view: its Azimuth Z ; the azimuth is counted positively in degrees in the retrograde direction for an observer upright at the origin of the coordinates.

This system is convenient for the location of an object to an exact date in a given place. However, it presents two important disadvantages:

because of the rotundity of the Ground, the coordinates of the same object at a given moment depend on the place of observation, contrary to the equatorial coordinated ;
the celestial objects are in relative displacement compared to a terrestrial observer because of the rotation of the Earth, and their two horizontal coordinates vary at a rate/rhythm which depends primarily on the variation of the objects concerned (cf sidereal Jour). Their follow-up by an instrument is thus less simple than with, for example, a equatorial mounting. Nevertheless much of instruments of observation azimuth mountings or altazimutales uses which is based on this frame of reference. It is the case of the last large telescopes brought into service (Télescopes Keck, Broad Very Telescopes,…).

In astronomical Navigation, it is in this system that one carries out measurements: basic measurement is that the height of a star (Sun, the Moon star). The calculation of the position requires the precise measurement of the hour of observation.

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