A point in the sky may most easily be fixed by an observer on the Earth with reference to his horizon. In the horizon coordinate system (see Figure 2), the position of the point is specified by its azimuth, the angle round from the northf point of the horizon (in the sense NESW) and by its altitude, the angle up from the horizon (positive if above the horizon, negative if below it). The positions of heavenly bodies, on the other hand, are very often described in the equatorial coordinate system (see Figure 3). Here the plane of the Earth's equator, extended to cut the celestial sphere, is used instead of the horizon, with the first point of Aires or vernal equinox taking the place of the north point of the horizon. A star's position is then given by the angle round from the vernal equinox along the equator (in the opposite sense to that in which it appears to move throughout the day) and the angle up from the equator (positive if to the north, negative if to the south). These coordinates are called the right ascension and declination respectively. Related to the right ascension is the hour angle which describes the angle along the equator from the observer's meridian. As the Earth rotates, so the star and the vernal equinox appear to move at the same rate, making the right ascension and the declination constants (but see routines PRCESS1 and PRCESS2). The hour angle, however, increases uniformly throughout the day from zero when the star crosses the meridian (moving westerly).