Astronomy and Cosmology

Author(s): L C Jain
Publisher: Rajasthan Prakrit Bharti Sansthan Jaipur

Page 28

cardinal directions. Above N in the northern direction is the Pole star (roughly) or the celestial pole at P, the south celestial pole being at P'. All are great circles with O as centre and O Nas radius, except the Co-D which is a small circle, with P as its Pole.
ZEZ' is the prime-vertical East, and Z WZ' is the prime-vertical West. The perpendicular to the polar axis and Pro'P is the celestial equator QER W. The secondary to this is the Pro' P'. Here is any star whose declination is o'r, the declination circle being Co-D.
The celestial equator cuts the ecliptic F-GV (path of the sun) at - and p, called the first point of Libra and the first point of Aries, or the equinoctial points. The position of the sun at these points make days and nights equal all over the earth. The pole of the ecliptic is denoted by K, near P.
The circle Pro’P may be called the hour circle as it measures the hour angle of the star's movement from the celestial meridian PZSP', always westwards, from Oh to 24h or 00 to 3600. Thus the declination o'and hour angle Z Po as substracted from 360° gives a system of celestial coordinates measured with the help of celestial equator and its secondary.
A second system of coordinates is set up from the ecliptic F-GP and its secondary through K H. Here Ho is called the celestial latitude of the star o, and the angle at K, measured eastwards from op between the secondaries K p and KoH to the ecliptic is celestial longitude of the star. plo' measures the right ascension of - 25.
The angle between the ecliptic and the equator is 234 degrees and is the same as the arc PK. The distance N P gives the altitude of the pole star or latitude of the observer. This is also equal to ZQ. Now F and G are solstitial points, when the sun is nearest or farthest from the earth,
When the star is at C, it is said to culminate or at upper meridian transit, at D it is at lower transit. o-B is the altitude of the star o, and Zo is its zenith distance.
25. Right ascension
coordinates
and declination forms another system of celestial