To observe the positions / direction and movement of the celestial bodies, an imaginary sphere of infinite radius is conceptualized having its centre at the centre of the earth. The stars are studded over the inner surface of the sphere and the earth is represented as a point at the centre. Figure 25.1 shows a celestial sphere and some principal parameters necessary to understand astronomical observation and calculations for determination of absolute direction of a line. The important terms and definitions related to field astronomy are as follows:
Figure 25.1 Celestial sphere and some principal parameters
Celestial sphere : An imaginary sphere of infinite radius with the earth at its centre and other celestial bodies studded on its inside surface is known as celestial sphere.
Great Circle(G.C) : The imaginary line of intersection of an infinite plane, passing through the centre of the earth and the circumference of the celestial sphere is known as great circle.
Zenith (Z) : If a plumb line through an observer is extended upward, the imaginary point at which it appears to intersect the celestial sphere is known as Zenith. The imaginary point at which it appears to intersect downward in the celestial sphere is known as Nadir (N).
Vertical circle : Great circle passing through zenith and nadir is known as vertical circle.
Horizon: Great circle perpendicular to the line joining the Zenith and Nadir is known as horizon.
Poles : If the axis of rotation of the earth is imagined to be extended infinitely in both directions, the points at which it meets the celestial sphere are known as poles. The point of intersection in the northern hemisphere is known as north celestial pole and that in the southern hemisphere as south celestial pole.
Equator : The line of intersection of an infinite plane passing through the centre of the earth and perpendicular to the line joining celestial poles with the celestial sphere.
Hour circle : Great circle passing through celestial poles is known as hour circle, also known as declination circle.
Meridian : The hour circle passing through observer's zenith and nadir is known as (observer's) meridian. It represents the North-South direction at observer station.
Altitude (h): The altitude of a celestial body is the angular distance measured along a vertical circle passing through the body. It is considered positive if the angle measured is above horizon and below horizon, it is considered as negative.
Azimuth (A) : The azimuth of a celestial body is the angular distance measured along the horizon from the observer's meridian to the foot of the vertical circle passing through the celestial body (Figure 25.2).