The angle measured clockwise from the Greenwich Meridian to the Hour Circle of the star.

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

Answer: (A)

This describes the Greenwich Hour Angle. It is the angular distance along the celestial equator from the Greenwich meridian to the star’s hour circle, measured westward (often described as clockwise when you picture the sky from the northern hemisphere). This reference to Greenwich time is why it’s used in navigation: GHA = Greenwich Sidereal Time minus the star’s right ascension, tying the star’s position to a fixed Earth reference.

Local hour angle, by contrast, is measured from your local meridian, not Greenwich, so it would yield a different angle. Sidereal hour angle is a related but separate convention tied to the star’s position relative to the vernal equinox, not to the Greenwich meridian. The term Hour Angle System isn’t a standard descriptor for this quantity.