Glossary term: Zenith

Description: The zenith is defined as the point directly above the observer. This makes the zenith an entity that is defined relative to the position of the observer, and the zenith for a person in, say, London would be different from that of a person in Beijing or Cape Town. Measured in angles, the zenith is 90 degrees from the observer's horizon. The zenith angle is the angular distance between a celestial object and the zenith. An object located at the zenith has a zenith angle of 0 degrees, and one on the horizon has a zenith angle of 90 degrees. The opposite of the zenith, namely the point straight below the observer, is called the observer's nadir. If Earth were a perfect sphere, the line linking an observer's zenith and nadir would pass through the center of the Earth, but as, in reality, the Earth is only approximately spherical, Earth's center is usually at some distance from that line.

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The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".