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Saints' days: anniversary days for Christian saints.
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scaphe dial: sundial where the hour lines are marked on a concave hemisphere or part hemisphere.
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sector: calculating instrument using pairs of lines on the faces of two hinged arms, see article on the sector.
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sexagenarium: an instrument in the form of a volvelle for planetary calculations.
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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.
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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.
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simple theodolite: surveying instrument for measuring horizontal angles, see article on the theodolite.
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