Declination (d) : The declination of a celestial body is the angular distance measured from the equator to the celestial body along the arc of an hour circle. It is considered positive in North direction and negative in South.
Ecliptic : The great circle along which the sun appears to move round the earth in a year is called the ecliptic.
Equinoctial points : The points of intersection of the ecliptic circle with the equatorial circle are known as equinoctial points. The point at which the sun transits from Southern to Northern hemisphere is known as First point of Aeries (g) and from Northern to Southern hemisphere as First point of Libra (W).
Right ascension : The right ascension of a celestial body is the angular distance along the arc of celestial equator measured from the First point of Aeries (g) to the foot of the hour circle. It is measured from East to West direction i.e., anti-clockwise in Northern hemisphere.
Prime meridian : Reference meridian that passes through the Royal Naval Observatory in Greenwich, England is known as prime meridian; it is also known as Greenwich meridian.
Longitude (l) : The longitude of an observer's station is the angular distance measured along the equator from the prime meridian to the observer's meridian. It varies from zero degrees to 180° E and 0° to 180° W.
Latitude (f): The latitude of an observer's station is the angular distance measured along the observer's meridian from the equator to the zenith point. It varies from zero degree to 90° N and 0° to 90° S.
Figure 25.3 Hour Angles
Hour angle (HA) : The hour angle of a celestial body is the angle at the equatorial plane measured westward from meridian to the hour circle passing through the celestial body (Figure 25.3).
Local hour angle (LHA): The angular distance of a celestial body measured westward from the point of intersection of the equator and the meridian of the observer to the foot of the hour circle passing through the celestial body.
Greenwich hour angle (GHA) : Angle at the equatorial plane measured westward from the prime (Greenwich) meridian to the hour circle through the celestial body.
Spherical triangle: Triangle formed by the intersection of three arcs of great circles (on the surface of the celestial sphere) is known as spherical triangle.
Note :
The dimension of the celestial sphere is so large that the position of the observer and the centre of the earth appears to be the same point.