|scaphe dial: sundial where the hour lines are marked on a concave hemisphere or part hemisphere.
|sector: calculating instrument using pairs of lines on the faces of two hinged arms, see article on the sector.
|sexagenarium: an instrument in the form of a volvelle for planetary calculations.
|shadow square: a form of geometrical quadrant, where two sides of a square are divided into equal parts and a plumb line or alidade from the opposite corner is used for measuring angles in terms of ratios (that is, tangents). Where a plumb line is used, one side has a pair of fixed sights. The name comes from the fact that a measurement of the altitude of the sun, expressed as a ratio, applied to the length of the shadow cast by an upright structure, yields its height. See also under astrolabe.
|sight: a device through which an object of interest can be viewed. Sights appear on surveying and astronomical instruments and in a more specialised form in the gunner's sight.
|simple theodolite: surveying instrument for measuring horizontal angles, see article on the theodolite.
|sinical quadrant: a quarter of a circle with a scale of degrees at its circumference which carries a pattern of criss-crossing vertical and horizontal lines. The ratio of the length of a given line to the quadrant's radius gives the sine or cosine of the corresponding angle.
|solar time: time measured directly from the position of the sun, see article on the sundial.
|solstice: either of two points on the ecliptic where the sun achieves its maximum declination. The term also refers to the two dates when the sun reaches this position in its annual cycle. The winter solstice corresponds to the shortest day, the summer solstice to the longest. Near these points the sun's declination changes only slowly, hence the etymological meaning of solstice as 'standing sun'.