Hour angle – choose the correct angular definition The angle between the observer’s meridian and the declination circle of a celestial body, measured at the celestial pole, is known as:

Introduction / Context:
Celestial coordinates come in pairs: equatorial coordinates (right ascension and declination) and horizontal coordinates (azimuth and altitude). The hour angle complements right ascension by measuring how far a body has moved westward from the local meridian.

Given Data / Assumptions:

• Declination circle: the great circle through the celestial poles and the body.
• Observer’s meridian: the great circle through the poles and the zenith/nadir.

Concept / Approach:
The hour angle H is the spherical angle at the celestial pole between the local meridian and the object’s declination circle. It increases westward with time, completing 24 h (360°) per sidereal day.

Step-by-Step Solution:
Identify the two great circles: meridian and declination circle of the body.The angle between them at the pole is, by definition, the hour angle.

Verification / Alternative check:
Local sidereal time equals the hour angle of the first point of Aries; this operational rule uses the same angle definition at the pole.

Why Other Options Are Wrong:

• Azimuth is measured in the horizontal plane from true north.
• Right ascension is measured eastward along the equator from Aries.
• Declination is angular distance north/south of the celestial equator.
• Amplitude refers to bearing at rise/set on the horizon.

Common Pitfalls:
Mixing horizontal and equatorial systems; forgetting that hour angle increases to the west.

Final Answer:
Hour angle