Field Astronomy - CIVIL ENGINEERING - SURVEY

Field    Astronomy    

 

DEFINITIONS   

ASTRONOMICAL TERMS   

 

1. The Celestial Sphere. 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 distances 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.

 

This imaginary sphere on which the stars appear to lie or to be studded is known as the Celestial Sphere.

 

The radius of the celestial sphere may be of any value-from a few thousand metres to a few thousand kilometres. Since the stars are very  distant from us, the centre of the earth may be taken as the centre of the celestial sphere.

 

2. The Zenith and Nadir. 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 point on 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.

 

3 The Celestial Horizon (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. Great circle is a section of a sphere when the cutting plane passes through the centre of the sphere.

 

4 The Terrestrial Poles and Equator. The 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 on earth. Is the radius of the plane which is at right angles to the axis of rotation The two poles are equidistant from it.

 

5. 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 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.

6. The Sensible Horizon. It is a circle in which a plane passing through the point of observation and tangential to the earth's surface (or perpendicular to the Zenith-Nadir line) intersects with celestial sphere. The line of sight of an accurately levelled telescope lies in this plane.

7. The Visible Horizon. It is the circle of contact, with the earth, of the cone of visual rays passing through the point of observation. The circle of contact is a small circle of the earth and 4, its radius depends on the altitude of the point of observation.

 

8. 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.

9. The Observer's Meridian. The meridian of any partycular point is that circle which passes through the Zenith and Nadir of the point as well as through the poles. It is thus a vertical circle.

 

10. The Prime Vertical. It is that particular vertical circle which is at right angles to the meridian, and which, therefore passes through the east and west points of the horizon.

 

11 The Latitude(theta) . 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 marked + or - ( N or S) according as the place is north or south of the equator The latitude may also be defined as the angle between the zenith and the celestial equator.

 

12 The Co-latitude (C). The co-latitude of a place is the angular distance from the zenith to the pele. It is the complement of the latitude and equal to (90°- theta).

 

13. The Longitude (phi). 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. The prime meridian The longitude of universally adopted is that of Greenwich. any place varies between 0° to 180", and is reckoned as (phi)° east of west of Greenwich.

 

14 The Altitude ( alpha ). The altitude of celestial or heavenly body (l.e., the sun or a star) is its angular distance above the horizon, measured on the vertical circle passing through the body.

 

15. 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, i.e., z=(90°- alpha).

 

16. The Azimuth (A). The azimuth of a heavenly body is the angle between the observer's meridian and the vertical circle passing through the body.

 

17. The Declination  The declination of a celestial body is angular distance from the plane of the equator, measured along the star's meridian generally called the declination circle, (i.e, great circle passing through the heavenly body and the celestial pole). Declination varies from 0° to 90°, and is marked ( + or -) according as the body is north or south of the equator.

 

18. Co-declination or Polar Distance (p). It is the angular distance of the heavenly body from the near pole. It is the Complement of the declination, ie.( p=90°-declination.)

 

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.