In the ________ of coordinates, the position of a star is uniquely specified by its right ascension and either its declination or its polar distance.

• A. Right Ascension System
• B. Equatorial Coordinate System
• C. Horizon Coordinate System
• D. Altitude-Azimuth System

Answer: (A)

In celestial coordinates, the equatorial coordinate system uses two angles to locate objects on the sky: right ascension and declination. Right ascension is like longitude on the celestial sphere, measured along the celestial equator from the vernal equinox, while declination is like latitude, measuring how far north or south an object is from the celestial equator. The polar distance is simply 90 degrees minus the declination. Because right ascension together with either declination or polar distance uniquely fixes a point on the celestial sphere, this combination defines the position unambiguously in the equatorial coordinate system. The horizon or altitude–azimuth systems, by contrast, depend on the observer’s location and time, so they aren’t described by right ascension and declination. And while right ascension is a key component, the standard name for the system is the equatorial coordinate system, not a separate “Right Ascension System.”