
right ascension: angle parallel to the celestial equator, measured eastwards from the spring equinox.

ring dial: simple form of altitude dial in the shape of a ring, for use in one latitude.

rule (or ruler): an instrument of multifarious functions in the form of a straight rod of metal, wood or other material; a folding rule is an alternative form with two or more jointed legs.

Saints' days: anniversary days for Christian saints.

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.
