Tuesday, December 11, 2007


HORIZONTAL COORDINATES SYSTEM - Azimuth, from the North point (red) also from the South point toward the West (blue). Altitude, green.
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 cannot (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.

The horizontal coordinates are:
altitude (Alt), sometimes referred to as elevation, 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, usually measured from the north point towards the east. In former times, it was common to refer to azimuth from the south, as it was then zero at the same time the hour angle of a star was zero. This assumes, however, that the star (upper) culminates in the south, which is only true for most stars in the Northern Hemisphere.

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 on the horizon, if at that moment its altitude is increasing, it is rising, if its altitude is decreasing it is setting. However all objects on the celestial sphere are subject to the diurnal motion, which is always from east to west, so the inherent cumbersome determination whether altitude is increasing or decreasing can be easily found by considering the azimuth of the celestial object instead.

if the azimuth is between 0° and 180° (north—east—south), it is rising.
if the azimuth is between 180° and 360° (south—west—north), it is setting.
There are the following special cases:
At the northpole all directions are south, so the azimuth is undefined there; no problem, the altitude of all celestial objects remains the same all the time, none ever rises or sets.
At the southpole all directions are north; same consideration.
At the equator objects on the celestial poles stay at one point on the horizon