
reduction compass: a drafting instrument with two pivoting arms which sits parallel to a drawing surface, held up by its fixed points. The arms have no scales but can be divided at any given position by moveable points.

Regiomontanustype dial: design of portable altitude dial adjustable for any latitude.

rete: skeletal star map representing the rotation of the heavens on an astrolabe. The rete normally features a projection of the ecliptic and pointers for prominent stars, and can be rotated over a chosen latitude plate. see article on the astrolabe.

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.

sinical quadrant: a quarter of a circle with a scale of degrees at its circumference which carries a pattern of crisscrossing 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'.

star pointer: point marking the position of a star, see article on the astrolabe.

string gnomon: the part of a sundial which casts the shadow, where this part is in the form of a taut string.

sun and moon dial: sundial that can also tell the time from the shadow cast by the moon, see article on the sundial.

sundial: instrument of many different forms for finding the time from the position of the sun, usually by measuring the position of a shadow, see article on the sundial.
