Question

The star Sirius has celestial coordinates 6 hours 45 minutes RA and -16 degrees 43 minutes declination. Explain what these coordinates mean and where you would expect to find it in the sky. [

Answer 1

The first coordinate is the right accession (RA) coordinate, it is like longitude measure in the sky. There is a 0 hour line marked by Sun at March equinox, it is then measured along the celestial equator which then gives the value of RA for a particular star. The second coordinate is the declination coordinate, it is like the latitude measure in the sky. It is same as the Earth's equator being projected to the celestial sphere. +ve declination means that the star is in Northern hemisphere and -ve declination means that the star is in Southern Hemisphere. Since here the star coordinate has a -ve declination, it implies that the star is in Southern Hemisphere. For RA based location estimation, we need to know from where the coordinates have been measured. But from declination we can be sure that the star is in the southern hemisphere.