Origins
The word 'nadir' is the conceptual and etymological twin of 'zenith,' and like its partner, it preserves a fragment of a longer Arabic astronomical phrase. It comes from Arabic 'naẓīr al-samt' (نظير السمت), meaning 'the counterpart of the path' or more precisely 'the point opposite the zenith direction.' Medieval Latin translators abbreviated this to simply 'nadir,' preserving only the first word, 'naẓīr' (نظير), meaning 'opposite' or 'counterpart.'
The Arabic root n-ẓ-r (ن-ظ-ر) means 'to look' or 'to observe,' and 'naẓīr' is formed from this root with the meaning of 'one who faces another' or 'a counterpart' — something that stands in a looking-at relationship with something else. In the original Arabic astronomical usage, the nadir was defined relationally: it was the point that 'looks at' the zenith from the opposite side of the sphere, the point directly beneath the observer's feet, as far below the horizon as the zenith is above it.
The word entered European astronomical vocabulary through the same channel as 'zenith' — the massive translation effort of the twelfth and thirteenth centuries, centered in Toledo and other Iberian cities, where Arabic scientific texts were rendered into Latin. Unlike 'zenith,' however, 'nadir' was transliterated with reasonable accuracy. The Arabic 'naẓīr' became Latin 'nadir' with only minor distortion, perhaps because the word was shorter and simpler, or perhaps because the scribes who handled this particular term were more careful than the one who mangled 'samt' into 'senit.'
Middle English
Chaucer used the word in his 'Treatise on the Astrolabe' (c. 1391), the same text in which he used 'zenith.' In Chaucer's usage, the nadir was a purely technical astronomical term with no figurative extension. The metaphorical sense — 'the lowest point of anything,' the depths of misfortune, failure, or degradation — developed gradually over the following centuries. By the seventeenth century, English writers were using 'nadir' figuratively, paired with 'zenith' to express the full range from highest success to deepest failure.
This figurative pairing of zenith and nadir became one of English prose's most durable rhetorical devices. Historians speak of empires at their zenith or nadir; biographers describe careers reaching a zenith before declining to a nadir; commentators describe markets, morale, and reputations in the same celestial terms. The paired metaphor works because it imports the elegant geometry of the celestial sphere into the messy domain of human affairs, lending an air of cosmic inevitability to the rise and fall of fortunes.
The full Arabic phrase 'naẓīr al-samt' is itself instructive about the relationship between the two words. 'Samt' (path, direction) is the same word that appears in the full form of zenith ('samt al-raʾs,' the path of the head) and in 'azimuth' (from 'al-sumūt,' the plural of 'samt'). The three English words — zenith, nadir, and azimuth — thus form a family derived from closely related Arabic astronomical phrases, all built around the concept of 'samt' (direction or path) in the geometry of the celestial sphere.
Greek Origins
The Arab astronomers who developed this terminology were building on and extending the work of Greek predecessors, particularly Ptolemy, whose 'Almagest' (itself an Arabic-mediated title, from 'al-majisṭī') was the foundational text of medieval astronomy. But the Arabic contributions went far beyond translation: astronomers like al-Battānī, al-Ṣūfī, and Ibn al-Haytham corrected Greek errors, developed new mathematical methods, and created the precise observational vocabulary that European astronomy would inherit. The words 'zenith' and 'nadir' are permanent monuments to this Arab scientific achievement, embedded in the everyday English vocabulary of anyone who speaks of reaching a peak or hitting bottom.