In astronomy and geography, a zenith angle of 0 degrees refers to a specific angular measurement of the observer’s position upward toward the zenith, which is directly above. A zenith angle of 0 degrees indicates that the line of sight or measured direction is perpendicular to the local horizontal plane. This means that the observed object or point of interest is positioned directly above the observer, without any deviation in the vertical plane.

In practical terms, a zenith angle of 0 degrees means the highest point in the sky that can be observed from a specific location, representing the direction of the zenith.

Zero elevation angle, similar to zenith angle, refers to the angular measurement of an object’s position above or below the horizon relative to the observer’s position. Specifically, a zero elevation angle indicates that the object is positioned exactly level with the horizon from the observer’s point of view.

This means that the altitude of the object is neither above nor below the eye level of the observer but rather aligned with the horizontal viewing plane. The zero elevation angle serves as a reference point for measuring the height or altitude of celestial or terrestrial objects relative to the horizon.

In standard astronomical and navigation conventions, the zenith angle is generally defined as positive when measured from the horizon toward the zenith (straight up).

However, in some specialized contexts or calculation methods, the zenith angle may be expressed as a negative value when measured from the zenith downward toward the horizon (directly down). This convention is less common and generally used in specific engineering applications where negative angles are needed for precise calculations or modeling purposes. In everyday usage and standard practices, zenith angles are generally considered positive values when measured from the horizon toward the zenith point directly above