What value corresponds to the sum of the star's hour angle and its right ascension?

• A. Local Sidereal Time
• B. Coordinated Universal Time
• C. Universal Time
• D. Greenwich Mean Time

Answer: (A)

Local Sidereal Time is the current angle (in hours) that tells which right ascension is on the local meridian. The hour angle tells how far the star is from that meridian, measured westward. To bring the star to the meridian you effectively rotate the sky by its hour angle, so the sum of the star’s hour angle and its right ascension equals the local sidereal time: LST = H + α (mod 24 hours). This is different from solar time standards like UTC, UT, or GMT, which are based on the Sun rather than the stars. For example, if a star has RA = 3h 20m and its hour angle is 2h 15m, the local sidereal time is 5h 35m.