The document defines several key astronomical terms: - The Celestial Sphere is an imaginary sphere with the observer at the center that stars appear to lie on. - The Zenith is the point directly above the observer, and the Nadir is the point directly below. - The Celestial Horizon is a great circle on the celestial sphere formed by the plane perpendicular to the zenith-nadir line. - The Terrestrial Poles are the points where the Earth's axis of rotation meets its surface, and the Terrestrial Equator is the great circle perpendicular to the axis of rotation. Extending the Earth's axis indefinitely produces the Celestial Poles and Celestial Equator on the celestial sphere.

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2. List of Astronomical Terms
The Celestial Sphere
The Zenith and Nadir
The Celestial Horizon
The Terrestrial Poles
and Equator
The Celestial Poles and
Equator
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3. DEFINITIONS
The millions of stars that we see in the sky on a clear cloudless
night are all at varying distances from us. Since we are concerned
with their relative distances rather than their actual distance
from the observer, it is exceedingly convenient to picture the
stars as distributed over the surface of an imaginary spherical
sky having its centre at the position of the observer. The
imaginary sphere on which the stars appear to lie or to be
studded is known as the Celestial Sphere
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4. Celestial Sphere
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5. The Zenith (z)is the point on the upper portion of the
celestial sphere marked by plumb line above the observer .
It is thus the point on the celestial sphere immediately
above the observer’s station .
The Nadir (Z’) is the lower portion of the celestial sphere
marked by the plumb line below the observer. It is thus the
point on the celestial sphere vertically below the
observer’s station .
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6. Zenith and Nadir
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7. It is the great circle traced upon the celestial sphere by
that plane which is perpendicular to the zenith – Nadir
line, and which passes through the centre of the earth.
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• The Celestial Horizon is a great circle & is a section of a
sphere when the cutting plane passes through the
centre of the sphere.

9. Terrestrial poles are the two points in which the earth’s axis of
rotation meets the earth’s sphere . The terrestrial Equator is
the great circle of the earth , the plane of which is at right
angles to the axis of rotation . The two points are equidistant
from it.
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10. If the earth’s axis of rotation is produced indefinitely, it will
meet the celestial sphere in two points called North and South
celestial Poles (P and P’). The celestial Equator is the great
circle of the celestial sphere in which it is intersected by the
plane of terrestrial equator.
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