The Greenwich Meridian is used as the origin for which hour angle?

• A. Local hour angle
• B. Sidereal hour angle
• C. Hour angle system
• D. Greenwich hour angle

Answer: (D)

The concept being tested is how hour angle is defined relative to a reference meridian. The Greenwich hour angle uses the Greenwich Meridian as its zero point, so the angle is measured westward from Greenwich to the object's hour circle. This makes it the standard reference when the origin is the Greenwich meridian, which is why it’s the best choice. Local hour angle uses the observer’s own meridian as the origin, so it’s different. Sidereal hour angle, on the other hand, is measured from the vernal equinox along the celestial equator, not from Greenwich. The term “hour angle system” isn’t a standard reference for a specific meridian.