__________is the hour angle of the First Point of Aries, and is equal to the hour angle plus right ascension of any star.

• A. Local Apparent Time
• B. Mean Solar Time
• C. Local Sidereal Time
• D. Greenwich Sidereal Time

Answer: (C)

Local Sidereal Time is the hour angle of the First Point of Aries. The First Point of Aries (the vernal equinox) has right ascension 0 hours, so its hour angle H equals LST − α = LST − 0 = LST. That means the hour angle of the vernal equinox is the local sidereal time.

For any star with right ascension α, its hour angle is H = LST − α. Rearranging gives LST = H + α. So the local sidereal time can be obtained by adding a star’s right ascension to its hour angle, which confirms why LST is both the hour angle of the First Point of Aries and equal to the hour angle plus the star’s right ascension.