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longitude: angle parallel to the equator to indicate position on the earth. The celestial longitude, however, is an angular distance parallel to the ecliptic.
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lunar dial: dial where the shadow is cast by moonlight instead of sunlight, see article on the sundial.
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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.
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magic square: square grid with a number in each cell for which the sum of each row, column and diagonal is the same.
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magnetic compass: instrument indicating directions by a magnetic needle, see article on the compass.
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magnetic deviation: the angle between geographical north and the direction indicated by a compass needle.
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magnitudes: numbers in a scale of the relative brightnesses of the stars, 1 being the brightest.
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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.
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mater: part of an astrolabe, see article on the astrolabe.
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mathematical compendium: compound instrument for performing various functions in practical mathematics.
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