What is the angle at which an observer's plumb line projects to the celestial sphere known as?

The angle at which an observer's plumb line projects to the celestial sphere is known as altitude. In the context of celestial navigation and astronomy, altitude measures the angle of an object above the observer's horizon. When observing celestial bodies, such as stars or planets, the altitude helps determine their apparent position in the sky relative to the horizon.

The concept of altitude is crucial because it allows observers to track the movement of celestial objects. A higher altitude indicates that a celestial object is positioned more directly overhead, while a lower altitude means the object is closer to the horizon. Therefore, altitude encompasses the critical relationship between the observer's location on Earth and their perspective of the celestial sphere.

The other choices represent different concepts, such as equatorial angle being associated with the celestial coordinate system, horizon angle relating to the angle formed at the horizon, and bearing being a direction or angle typically used in navigation concerning north. These terms do not specifically pertain to the projection of an observer's plumb line onto the celestial sphere as clearly as altitude does.