The zenith is the vertical direction of a location that is extended upwards . The opposite direction is called nadir ("foot point"). This axis defines the horizon plane as its normal . Together with the celestial poles , the zenith is the basis for astronomical coordinate systems .
The name comes from the Arabic سمت الرأس / including ar-raʾs / 'direction of the head'. Spelling mistakes and incorrect readings resulted in medieval texts from including the spelling cemt or zemt and from this cenit or zenit .
- Wolfgang Torge : Geodesy. De Gruyter, 2003, ISBN 3-11-017545-2 , limited preview in the Google book search.
- Collins English Dictionary - Complete & Unabridged. 10th edition, quoted from Dictionary.com .
- Wolfgang Torge: Geodesy. De Gruyter, 2003, ISBN 3-11-017545-2 , p. 90 .