## THE CELESTIAL SPHERE

It is a sphere with an infinite radius and the center same as the Earth’s center and all heavenly bodies appear to lie on the inner surface of the sphere.

All heavenly bodies appear to rise in the east and set in the west.

**Celestial pole – **If we extend the axis of the Earth in either direction it will intersect the celestial sphere at the two points called the celestial pole.

**Celestial meridians **– Semi great circles joining celestial poles are called the celestial meridian.

**Celestial equator or Equinoctial** – The plane of the equator is extended all around it will intersect the celestial sphere at many points. The locus of the points forms a great circle and is called celestial equator or equinoctial.

**Ecliptic –** The path of the sun in the celestial sphere is called the ecliptic. The ecliptic is inclined at an angle of 23 and a half with the equinoctial because the Earth’s axis is tilted by the same amount as normal to the plane of the Earth’s orbit.

The ecliptic and equinoctial intersect at two points – **FIRST POINT OF ARIES **and **FIRST POINT OF LIBRA.**

## HORIZON SYSTEM-

**Observer’s Zenith** – is the point on the celestial sphere vertically above the observer i.e. the point at which a straight line from the center of the Earth through the observer meets the celestial sphere.

**Observer’s Nadir **– is the point on the celestial sphere vertically opposite his Zenith.

**Vertical circle **– The great circle passing through Zenith.

**Principal vertical circle (observer’s meridian) **– The vertical circle passing through poles is called a** **principal vertical circle.

**Prime vertical circle –** It is the vertical circle passing through an east-west point.

**Celestial Horizon or Rational Horizon** –It is a great circle every point on it is 90 degrees away from Zenith.

**Visible Horizon –** It is a small circle on the surface of the Earth. Where the sky and the sea appear to meet.

**Sensible horizon** – It is the plane along with the Horizon sight of the observer parallel to the celestial horizon.

**The altitude of the heavenly body**– It is the angular distance from the horizon to the heavenly body along the vertical circle passing through the heavenly body.

**Azimuth **– It is the angle at Zenith between the observer’s meridian and the vertical circle passing through the heavenly body.

With altitude and azimuth of a heavenly body, the heavenly body can be indicated in celestial sphere w.r.t observer.

**Zenith distance **– It is an angular distance from the zenith to the heavenly body along the vertical circle passing through the heavenly body.

## EQUINOCTIAL SYSTEM

**Sidereal hour angle (SHA)-**

It is an angle at the pole or the angular distance along the equinoctial between the meridian passing through the first point of Aries and meridian passing through the heavenly body measured westward from the meridian through the first point of Aries.

It varies from zero to 360 degrees or 0 hrs. to 24 hrs.

**Right Ascension(RA) –** It is an angle at the pole or angular distance along the equinoctial between the meridian passing through the first point of Aries and meridian through the heavenly body measured eastwards from the meridian through the first point of Aries.

**RA = 360 degree – SHA**

**Declination **– It is the angular distance from the equinoctial till the heavenly body along the meridian measured from zero degree to 90 degrees north or south. If the heavenly body is above equinoctial it has north declination and below equinoctial is south declination.

With SHA and declination, we can indicate the heavenly body in space. This position is fixed in space.

**Px = 90 degree – declination **

**Where Px = polar distance**