## ASTRONOMICAL TERMS

**THE CELESTIAL SPHERE**is an imaginary sphere upon the surface of which all the stars in the sky appear to be studded to an observer stationed at its centre.

**THE ZENITH**is the intersection of a vertical line through the observer's station with the upper portion of the celestial

sphere. It is the point on the celestial sphere immediately above the observer's station.

**THE NADIR**is the intersection oI a vertical line through the observer's station with the lower portion oi the celestial sphere. It is the point on the celestial sphere vertically below the observer's stations.

**THE CELESTIAL HORIZON**(also called TRUE or RATIONAL HORIZON) is the great circle in which a plane at right angles to the Zenith and Nadir line and passing through the centre or the earth intersects the celestial sphere. The Zenith and Nadir are the poles oi the celestial horizon.

**THE SENSIBLE Horizon**is the circle in which a plane tangent to the earth's suriace (or at right angles to the Zenith and Nadir lines) and passing through the point oi observation intersects the celestial sphere. The line or sight of an accurately

levelled telescope lies in this plane.

**THE VISIBLE HORIZON**is the circle or contact oi the earth and the cone of visual rays passing through the point of observation.

**THE TERRESTRIAL**Equator (or simply called EQUATOR) is the great circle or the earth, the plane oi which is perpendicular to the axis or rotation (polar axis).

**THE POLAR AXIS**is the diameter about which the earth spins. The extremities of the axis or rotation (polar axis) of the earth, are known as the poles, they are distinguished as the

**NOHTH POLE**and the

**SOUTH POLE**.

**THE CELESTIAL EQUATOR**is the great circle in which the plane at the equator cuts the celestial sphere.

**THE CELESTIAL POLES**are the points of intersection of the axis of the earth (or the polar axis) when produced with the celestial sphere.

**THE CELESTIAL MERIDIAN**is the great circle in which the plane passing through the celestial poles intersects the celestial sphere.

**THE MERIDIAN**of a place or an observer is the great circle passing through the zenith, and nadir and the poles.

**THE ECLIPTIC**is the great circle which the Sun appears to trace on the celestial sphere With the earth as a centre in the course of a year. The plane of the ectiptic is not coincident with the plane of the equator, the angle between them being known as the OBLIQUITY OF THE ECLIPTIC. Its value is about 23° 27'.

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