Non-SI unit of plane angle: \(1^{\prime\prime} = \frac{1}{3600}\ ^{\unicode{x26ac}} = 4.848\ 137\,...\,\times\ 10^{-6}\ \rm{rad}\).
Sources: Green Book, 2nd ed., p. 113 [Terms] [Book] PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 991 [Terms] [Paper]