What is the angular distance, measured westward from the observer's celestial meridian, called in the equatorial coordinate system?

In the equatorial coordinate system, the angular distance measured westward from the observer's celestial meridian is referred to as the hour angle. The hour angle represents the time since the last transit of a celestial object across the observer's meridian, expressed in angular measurement. As the Earth rotates, celestial objects rise in the east and set in the west, and the hour angle helps to determine their position in relation to the meridian at any given time.

The hour angle is particularly useful for astronomers and navigators, as it allows them to calculate the time until a star or other celestial body reaches its highest point in the sky (the culmination). This concept is foundational for understanding celestial navigation and the movement of astronomical objects as it relates to the observer's location and local time.

In contrast, hour circles are great circles on the celestial sphere that pass through the celestial poles and the given star, while the celestial meridian refers to a specific line on the celestial sphere that is based on the observer's longitude. Local sidereal time is a timekeeping system that relates to the position of stars and is not directly about measuring angular distances related to a specific celestial object.