N geographic latitude. Equatorial system of coordinates. The horizon system is shown for orientation. For a star in the southwestern part of the sky, the coordinates locating its position are called right ascension and declination. Right ascension is measure counterclockwise in units of sidereal time from the vernal equinox along the celestial equator to the foot of the hour circle passing through the star.
Declination is measured in degrees from the celestial equator along the hour circle to the star.
As the Earth rotates, the hour angle of the vernal equinox continually increases, repeating after a 360 o rotation of the Earth or 24 sidereal hours. Thus the local hour angle of the star, which continuously changes over time, can be found from the sidereal time and the star's right ascension. Why the solar day is longer than the sidereal day. With an observer at position 1, the Sun is on the observer's celestial meridian. After a 360 o rotation relative to the stars, Earth has moved about 1 o along its orbit and the observer will be at position 2, where the Sun is now 1 o east of his or her celestial meridian.