Nonsetting and Nonrising Stars

As the Earth revolves on its axis from west to east, the celestial sphere appears to rotate in the opposite direction. And so we see the stars rising in the east and setting in the west. But all stars do not follow this routine. To a viewer north of the equator, the pole star (Polaris) in the north (almost coinciding with the north celestial pole) appears to remain stationary in the sky without ever setting or rising. Other stars appear to move in an anticlockwise direction around it. As a result, depending upon the latitude of the observer, some stars in the northern sky never seem to set while others, in the sourthern sky, never seem to rise at all. From the same latitude, the rest of the stars rise (in the east, northeast or southeast depending upon the declination of the star), culminate, and then set (in the west, northwest or southwest respectively). To an observer in southern latitudes, facing south, the stars appear to move in a clockwise direction around the imaginary south celestial pole. (There is no 'fixed' star corresponding to the south celestial pole in the southern sky.)

If we know the latitude of any place, we can find out which stars are non-setting and which ones are non-rising. New Delhi, for example, has a latitude of 28°N (approx.). If we subtract 28° from 90° we get 62°. This means that from New Delhi, any star with a declination of more than +62° is non-setting. That is, if we look at such a star it would appear to simply go round and round the Polaris without ever dipping below the horizon. Of course, we'll see the non-setting stars only during the night as daylight would blot them out during the day Similarly, all stars with declination less than -62° (in the southern sky) would be non-rising for observers in New Delhi, for they would always lie below the southern horizon.

If we are an observer from Kanyakumari (lat. 8°N), the situation would be quite different. From here we will see the Polaris almost on the northern horizon. (In fact, it would be only 8° above the horizon where atmospheric haze almost blots it out.) So, hardly any star in the northern sky would be non-setting from here, and there would be few non-rising stars below the southern horizon. For observers south of the equator, the non-setting stars would be seen in the southern sky and the non-rising stars would be below the northern horizon.


If we are at the equator, all the stars will appear to move overhead, parallel to the east-west direction as they rise and set. But if we happen to be at the north or south pole, the stars would appear to go round in circles around a common centre above our head. The motion will be in a clockwise direction over the south pole and in an anticlockwise direction over the north pole.

Was this article helpful?

0 0

Post a comment