
lunar dial: dial where the shadow is cast by moonlight instead of sunlight, see article on the sundial.

lunar volvelle: volvelle indicating the age of the moon, which typically converts between the time indicated by the shadow of the moon on a sundial and solar time.

magic square: square grid with a number in each cell for which the sum of each row, column and diagonal is the same.

magnetic compass: instrument indicating directions by a magnetic needle, see article on the compass.

magnetic deviation: the angle between geographical north and the direction indicated by a compass needle.

magnitudes: numbers in a scale of the relative brightnesses of the stars, 1 being the brightest.

mariner's astrolabe: altitudemeasuring instrument for finding latitude at sea, having a heavy brass or bronze ring, suspended by a shackle and ring, with a degree scale and a centrallypivoted alidade.

mater: part of an astrolabe, see article on the astrolabe.

mathematical compendium: compound instrument for performing various functions in practical mathematics.

mathematical instruments: collections of instruments of assorted kinds for performing functions in practical mathematics.

measuring rod: an instrument used to measure distances or depths. See also gauging rod.

measuring rods: see measuring rod.

meridian: the plane containing the observer and both celestial poles, or containing the observer, the zenith and the pole, or, more colloquially, the northsouth line at a particular position on the earth.

meridian circle: circular scale of degrees which in use is aligned with the meridian.
