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| Figure shows Celestial Sphere |
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.
Visible horizon
It is a circle of contact, with the earth, of the cone of visual rays passing through the point of observation.
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| Figure shows Zenith,Nadir, Sensible horizon, Visible horizon and Celestial horizon |
Terrestrial poles are the two points in which the earth’s axis of rotation meets the earth’s sphere.
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 P’).
The celestial equator is the great circle of the celestial sphere in which it is intersected by the plane or terrestrial equator.
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| Figure shows Terrestrial poles, Terrestrial equator ,Cellestial poles and Celestial Equator |
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.
Prime vertical
It is the particular vertical circle which is at right angles to the observers meridian that passes through the east & west points of horizon.
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| Figure shows Vertical circle, Prime Vertical and Observer's Meridian |
Observers Meridian or Reference Meridian
Observers meridian is a special meridian which passes through the zenith and nadir of the point as well as through the Celestial poles.
It is the angular distance of any place on the earth’s surface measured towards north or south of the equator. It is also defined as the angle between the Zenith and the Celestial equator or Nadir and Celistial equator. Range 0°to 90°
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| Figure shows Azimuth 'A' and Altitude 'α' |
The altitude (α)
The altitude of celestial or heavenly body (i.e., a sun or star) is its angular distance above or below the horizon, measured on the vertical circle passing through the body.
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.
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The declination of a celestial body is angular distance from the plane of the equator, measured along the star’s meridian.Declination varies from 0° to 90°, and is marked + or – according as the body is north or south of the equator. Great circle of celestial sphere passing through Celestial body as well as celistial poles.
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| Figure Shows Declination circle or 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.
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| Figure shows 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.
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