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radio latino: instrument for measuring angles in surveying and gunnery, see article on the radio latino.
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ramming rods: long rods, often in wood, intended for use in gunnery and used to push the projectile inside a cannon, or to compress the powder in an arquebus.
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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.
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Regiomontanus-type dial: design of portable altitude dial adjustable for any latitude.
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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.
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right ascension: angle parallel to the celestial equator, measured eastwards from the spring equinox.
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ring dial: simple form of altitude dial in the shape of a ring, for use in one latitude.
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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.
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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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