zenith [LME] Like its opposite, nadir [LME], zenith was originally an astronomical term borrowed from Arabic, in this case from samt ar-ra's, ‘path over the head’. In astronomy the zenith is the point in the sky immediately above the observer, and also the highest point reached by a particular celestial object, when it is at its zenith. The modern general sense of this developed from the astronomical use in the early 17th century. The nadir is the point in the sky immediately below the observer, and comes from Arabic nazīr, meaning ‘opposite [to the zenith]’. Its general sense, ‘the lowest or most unsuccessful point’, also developed in the early 17th century.