The angle measured clockwise from the Vernal Equinox to the Hour Circle of the star.

• A. Greenwich hour angle
• B. sidereal hour angle
• C. Hour Angle System
• D. Local hour angle

Answer: (B)

When you map positions on the celestial sphere, different hour angles are defined from different fixed reference lines. The angle described here is measured clockwise from the vernal equinox to the star’s hour circle, which is exactly the definition of the sidereal hour angle. It uses the vernal equinox as the reference point and moves to the star along the celestial equator, independent of where you are located. Since right ascension is measured eastward from the vernal equinox, the sidereal hour angle relates to it by SHA = 360° − RA (mod 360°). This makes SHA a fixed celestial reference for the star's position.

The other options refer to different references: Greenwich hour angle uses the Greenwich meridian, not the vernal equinox; local hour angle uses the observer’s local meridian; and “Hour Angle System” isn’t a standard term.