In astronomy, what does altitude measure?

Altitude in astronomy measures the vertical angle from the observer's local horizon to a celestial body, such as a star or planet. This is typically expressed in degrees, where a celestial object at the zenith is 90 degrees above the horizon, and an object on the horizon is at 0 degrees. Understanding altitude is crucial for locating objects in the night sky since it directly relates to an observer's geographic position and the local vertical axis.

The other answers, while related to celestial coordinates, describe different measurements. The angular distance from the celestial equator pertains to declination, which is analogous to latitude but applied to the celestial sphere. Distance to the north celestial pole refers to a specific point in the sky and is measured by right ascension and declination rather than altitude. Lastly, distance along the celestial equator involves tracking movement in a plane perpendicular to the celestial poles and does not address the relative vertical position of celestial objects above or below the horizon.