Equatorial frame of reference

In Astronomy, to mitigate the defects of the horizontal Frame of reference, one uses the equatorial frame of reference . It allows, for example, to locate the position of a star in the sky whatever the place and the date.

This system uses as datum-line projection on the celestial Sphère of the equator of the Ground. One calls this projection the celestial equator. The point of reference of this circle is the “vernal Point” located close to the constellation of the Poisson. This circle is divided of 24 hours (either of divisions of 15 degrees each one). The angle measured between the projection of the object on this circle and the vernal point (while leaving towards the east this point) is called L Right ascension . It is thus expressed in hours, minutes, seconds.

The angle between this celestial equator and the object are called the variation . It is measured in degrees, positive for the objects located in the northern and negative hemisphere for the others. The axis of the poles of this system thus coincides with the axis of rotation of the Earth.

For example, α Ursae Minoris, which is practically located in the axis of rotation of the Earth has a Right ascension of 2:31 min and a variation of 89°15'. Bételgeuse, the Géante red of the left shoulder of Orion is located at 5:55 min right ascension and 7°24' of variation. To note that the belt of Orion is located very close to the celestial equator which divides the constellation in two.

This system is very much used for the location since the Earth of the celestial objects apart from the Solar system (star S, Galaxie S,…), relatively motionless. In the case of objects which have a own Mouvement detectable, like the Comet S, or the stars close, one associates with the coordinates and at the time, the variations of the right ascension and the variation during time. These variations are then expressed in seconds (of arc) a second…

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