Celestial Navigation Part 1

Celestial Sphere :-
Imaginary sphere around the Earth, of infinite radius, having its centre as the centre of the Earth. All celestial bodies, including the Sun and Moon, are projected to lie on this sphere.

Celestial Poles:-
Two points where the axis of the Earth when projected meets the celestial sphere. One of them is called the North pole and the other the South pole.

Equinoctial:-
Great circle on the celestial sphere, whose plane is perpendicular to the axis of the Earth or the line joining the celestial poles.

Declination Circle:-
Small circle on the celestial sphere, whose plane is parallel to the Equinoctial.

Celestial Meridian:-
Semi-great circle on the celestial sphere, whose plane passes through the celestial poles, and is perpendicular to the Equinoctial and Declination circles.

Declination:-
Arc of a celestial meridian or angle at the centre of the celestial sphere, contained between the Equinoctial and the Declination circle passing through a body. It is measured North or South of the Equinoctial, starting from 0 at the Equinoctial upto 90 at the celestial pole.

Hour Angle:-
Arc of the Equinoctial or angle at the celestial pole, contained between any two celestial meridians.
Prime Meridians:-
1. Greenwich meridian projected on the celestial sphere.
2. Meridian of an observer projected on the celestial sphere.
3. Celestial meridian passing through the First Point of Aries.

Greenwich Hour Angle (GHA)
Arc of the Equinoctial or angle at the celestial pole, contained between the celestial meridian of Greenwich and the celestial meridian of a body, measured from 0 to 360 westward (clockwise) from the Greenwich meridian.

Local Hour Angle
Same as GHA except that Greenwich is replaced by the Observer.