The Night Sky


What are circumpolar stars?

Circumpolar stars (also called non-setting stars) are permanently neither visible above the horizon rise nor set. The number and type of circumpolar stars will naturally depend on the location of the observer. At the Equator, the northern and southern celestial poles would appear on the horizon, and no stars would be circumpolar. For intermediate latitudes, some stars would be circumpolar, some would rise or set and some would never be seen at all.



How do astronomers locate and refer to individual stars in the sky?

The coordinates that determine the position of a star on the celestial sphere are “Right Ascension” (R.A) and “Declination” (D.). These are polar coordinates which specify of reference of the star and determine the celestial longitude and latitude at which the star can be located.

What is Right Ascension?

The Right Ascension reference gives the celestial longitude of a body Zero Right Ascension reference is the point where the celestial equator and the ecliptic intersect. In other words, it is the vernal equinox or the point at which the Sun moves into the northern half of the celestial sphere. Considering that one circuit of the celestial sphere is equivalent to 3600 in 24 hours, we can say that 150 of Right Ascension are equivalent to 1 hour of time. The meridians at intervals at 150 are therefore called Hour Circles for the same reason. The Right Ascension reference helps the observer to align himself in the correct direction on the horizontal plane.

What is Declination?

Once the observer has aligned himself in the correct direction on the horizontal plane through the Right Ascension reference, he can locate the star by aligning himself along the vertical plane. In other words, together with Right Ascension, Declination defines the position of a body in the sky. Declination is the extent to which the star is above the celestial equator. Declination can therefore vary from 0 to 900.




What is correction for planetary parallax?

The coordinates (R.A. & D.) of astronomical bodies on the Celestial Sphere are theoretically with reference to the centre of the Earth. The observations of an observer located on the Earth’s surface have necessarily to be corrected by the distance between the Earth’s centre, is called correction for planetary parallax.