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: altitude-measuring 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 centrally-pivoted alidade.