Local Sidereal Time is the hour angle of the First Point of Aries.

• A. True
• B. False
• C. The hour angle of the Vernal Equinox
• D. The hour angle of the North Celestial Pole

Answer: (A)

Local sidereal time is the angle between your local meridian and the vernal equinox point on the celestial sphere. The vernal equinox is the First Point of Aries, and it defines zero right ascension. Because hour angle measures the position of a given point in terms of the local meridian, the hour angle of that reference point equals the local sidereal time. Therefore, Local Sidereal Time is the hour angle of the First Point of Aries.

(Note: the hour angle of the North Celestial Pole isn’t used to define LST, and the hour angle of the Vernal Equinox describes the same quantity since the vernal equinox and the First Point of Aries are the same point.)