1. Celestial sphere :
It is an imaginary sphere on which the stars appear to lie or to be studded is known as the Celestial sphere.
2. Zenith (z) :
It is the point on the upper portion of the celestial sphere marked by plumb line above the observer. It is the point on the celestial sphere immediately above the observer's station.
3. Nadir (Z', or, N):
It is the point on the lower portion of the celestial sphere marked by plumb line below the observer. It is the point on the celestial sphere vertically below the observer's station.
It is also called true or Rational horizon or geocentric horizon. 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.
5. The terrestrial poles and equator :
The terrestrial poles are the two points in which the earth's axis of notation meets the earth's sphere.
The terrestrial equator is the greet circle of the earth, the plane of which is at right angles to the axis of notation. The two poles are equidistant from it.
6. The celestial poles and equator :
If the earth's axis of rotation is produced indefinitely, it will meet the celestial sphere in two points called the North & South celestial poles (P and
The celestial equator is the great circle of the celestial sphere in which it is intersected by he plane or terrestrial equator.
7. Sensible horizon:
It is a circle in which a plane passing through the point of observation and tangential to the earth's surface intersects with celestial sphere. The line of sight of an accurately leveled telescope lies in this plane.
8. Visible horizon :
It is a circle of contact, with the earth, of the cone of visual rays passing through the point of observation.
9. Vertical circle :
A vertical circle of the celestial sphere is great circle passing through the zenith and nadir. They all cut the celestial horizon at right angles.
10. The Observers Meridian:
The meridian of any particular point is that circle which passes through the zenith and nadir of the point as well as through the poles.
11. Prime vertical:
It is the particular vertical circle which is at right angles to the meridian and which therefore passes through the east & west points of horizon.
12. Latitude (? ):
It is the angular distance of any place on the earth's surface north or south of the equator, and is measured on the meridian of the place. It is also defined as the angle between the zenith and the celestial equator.
The co-latitude of a place is the angular distance from the zenith to the pole. It is the complement of the latitude and equal to (900- ? ).
14. The longitude (?):
The longitude of a place is the angle between a fixed reference meridian called the prime or first meridian and the meridian of the place.
15. The altitude (?):
The altitude of celestial or heavenly body (i.e., a sun or star) is its angular distance above the horizon, measured on the vertical circle passing through the body.
16. The co-altitude or zenith distance (z):
It is the angular distance of heavenly body from the zenith. It is the complement of the altitude.
17. The Azimuth:
The azimuth of a heavenly body is the angle between the observer's meridian and the vertical circle passing through the body.
18. The Declination:
The declination f a celestial body is angular distance from the plane of the equator, measured along the star's meridian generally called the declination circle. Declination varies from 00 to 900, and is marked + or - according as the body is north or south of the equator.
19. Hour circle:
Hour circles are great circles passing through the north and south celestial poles. The declination circle of a heavenly body is thus its hour circle.
20. The hour angle:
The hour angle of a heavenly body is the angle between the observer's meridian and the declination circle passing through the body. The hour angle is always measured westwards.