The angle between the observer’s meridian and declination circle of a heavenly body, is known as A. Hour angle B. Azimuth C. Right ascension D. Declination

The correct answer is A. Hour angle.

Hour angle is the angle between the observer’s meridian and the hour circle of a heavenly body. It is measured eastward along the celestial equator from the observer’s meridian to the hour circle, and is equal to the local sidereal time minus the right ascension of the body.

Azimuth is the angle between the north celestial pole and the projection of a celestial object’s position onto the horizon. It is measured eastward from north, and ranges from 0° to 360°.

Right ascension is the angular distance of a celestial object east of the vernal equinox, measured along the celestial equator. It is measured in hours, minutes, and seconds, and is equivalent to the hour angle of the object at the vernal equinox.

Declination is the angular distance of a celestial object north or south of the celestial equator. It is measured in degrees, minutes, and seconds, and can be positive or negative.