Yesterday we discussed how the letter dalet alternates with zayin in the words נזר and נדר - both meaning types of vows. Steinberg connects those two verbs with two additional ones meaning "to guard" - נטר and נצר. He says they all have a sense of "placing a boundary".
There is an English word that comes from the same family of verbs, or at least the last two. Nadir means "the lowest point", coming from astronomy, where it means: "point on the celestial sphere directly below the observer, diametrically opposite the zenith."
Klein gives the following etymology:
Fren. nadir, from Arab. nazir in the term nazir assamt (lit.: 'the point opposite to the zenith'), from nazara (=he looked at; he considered, examined), which is related to Hebrew נצר ( = he watched over, guarded.)
The homonym nadir meaning "rare", has a different origin:
From Arab. nadr ( =rare), from nadara (= was rare) from nadara in the sense 'fell away, fell', which is related to Aram.-Syr. נדר ( = it sloped, was declivitous). See מדרון.
By the way, zenith also comes via Arabic, but originally derives from Latin. Klein writes:
Fren. zenith, from Sp. cenith, from VArab semt, corresponding to classical Arab samt (=way, path), abbr. of samt ar-ra's (= way over the head), from Latin semita (=path).
This may be the origin of sentry or sentinel - the semita was the sentry's beat. Somehow we got back to guarding again...