What does zenith distance refer to in astronomy?

Zenith distance specifically refers to the angular measurement from the zenith, which is the point in the sky directly above an observer, to a celestial body, such as a star or planet. This angular distance is important in astronomy as it helps to define the position of celestial objects in the sky relative to a specific observer's location.

By measuring the zenith distance, astronomers can derive other valuable information, such as altitude and azimuth, which are critical for locating astronomical bodies. Zenith distance varies depending on the observer's location on Earth and the time of observation, making it a key factor in celestial navigation and observational astronomy.

In contrast, the other options describe different concepts: the angle between two celestial bodies does not address the zenith specifically, the distance from Earth to the zenith is not relevant as the zenith is a conceptual point at the observer’s location, and the radius of the celestial sphere is a geometric concept unrelated to angular measurements from an observer’s zenith.