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In astronomy, zenith indicate the point of celestial sphere representative of the ascending vertical direction, in a place given (in opposition to to nadir), or more simply the vertical point above our head.
With the illustrated direction, the zenith is the degree more raised, the apogee.
Certain telescopes are conceived with mirrors liquidate (with mercury by ex.) in rotation. Owing to the fact that they must remain permanently with the vertical, this kind of telescope has a field of observation restricted with the zenith.