Question

Betelguese has celestial co-ordinates with Right Ascension approximately 6 hours and Declination approximately 0 degrees. What is the sidereal time when Betelguese rises ?

Answer 1

A solar day is one in which the earth rotates once around the Sun and the Sun returns to the same place in the sky. Sidereal time is measured relative to the positions of all other stars in the sky.

A sidereal day is the time every star takes to travel around and reach the same position in the sky. It is slightly shorter than a Earth day, it is 23 hours, 56 minutes and 4.1 seconds. A sidereal day is divided into 24 sidereal hours, which are in turn divide into 60 sidereal minutes and also into seconds.

Every objects in the sky are located by their celestial positions. This position is given as “Right Ascension” and “Declination”. Right ascension (RA) is like longitude. It locates where a star is along the celestial equator. The Sidereal time is nothing, but the right Ascension.

Hence the sidereal time of Betelgeuse is also 6 hours.