Message #50051

[Bianca]

M





Mach bands: (pron. mak) subjective light and dark bands which an observer sees when looking at a black-white edge. They are produced by the brain's visual processing (i.e. they are not real) and have the effect of sharpening up edges. First described by the German physicist Ernst Mach (1838-1916).

magnetic variation {magnetic declination, magnetic deviation}: Note, the use of the term magnetic declination is best avoided because of confusion with the other types of declination. It is the angle between the true N pole and the magnetic N pole. At present in the UK, the magnetic pole is very approximately 3º W of true north, and decreasing by about 12' annually. See Appendix IX for more detailed and historical values. There can be large local variations to the general values, some of which can be found mapped on navigational charts. The use of a magnetic compass for aligning a permanent dial is not recommended, even if due corrections are made, as the presence of steel or magnetic rocks will cause very local variations.

manaeus: (pron. man-ay-us) the circle of months which formed part of the ancient orthographic spherical projection used by late Middle age diallists. It establishes the sun's declination.

maquette: a sculptor's small preliminary model. The term is used to describe small mock-ups of three-dimensional dials.

Martinmas: St Martin's day, on 11th November. It is one of the cross-quarter days.

mass dial: see Dial types (mass dial).

mater: the heavy disk which forms the base of an astrolabe.

mean solar day: the time between successive transits of the fictitious mean Sun (i.e. an imaginary sun which appears to circle around the celestial equator at a constant rate equal to the average rate of the Earth's real rotation). The basis of civil time keeping.

mean time: see Time (types of).

mean local time (or local mean time) : see Time (types of).

Mercator projection: the most common projection used to produce a 2-D map of the globe. Developed by Gerardus Mercator in Belgium, 1586. It has straight meridians and parallels of latitude that intersect them at right angles. Scale is true at the equator or at two standard parallels equidistant from the equator. The Transverse Mercator projection is obtained by projecting the sphere onto an enclosing cylinder tangent to a central meridian. This is the projection used for Ordnance Survey maps of the UK.

merkhet: a transit instrument from ancient Egypt, consisting of a horizontal "L" shaped stone with a plumb-bob supported from the short vertical arm. It was used in conjunction with a bay.

meridian: the great circle (or, more usually, half of a great circle) passing through the N and S poles. The same as a line of longitude. The term is sometimes used to mean the meridian line passing through the observer's location, or its representation on the dial face.

meridional: south-facing (e.g. a direct-south dial). In more general usage, it generally means of, or from, the south.

meridian line: see Dial Types (noon line) for the lines inscribed in the floors of Renaissance cathedrals, etc.

metonic cycle: a cycle of 19 years (or 235 lunar months) over which the Sun and the Moon return to the same relative positions amongst the constellations. It was discovered by the Greek astronomer Meton c.433 BC and determines the epact number and the Golden Number. Actually, the moon runs 1½ hours slow over this period, or one day over 312.7 years. This fact has to be included in the calculations for Easter.

midnight: strictly, the time when the Sun achieves its most negative altitude (or, equivalently, when its azimuth is ±180º). More loosely defined as half-way between sunset and sunrise or, with even less accuracy, 12 hours after local noon.

midsummer, midwinter (~ day): the same as summer or winter solstice. Note that Midsummer (with capital M) is a legal term for the Quarter Day on June 24.

mil: unit of angular measurement used in some military equipment, e.g. rangefinders, theodolites. 6400 mils = 360º. Beware possible confusion with use as a linear measurement of 1/1000 inch used by engineers (particularly in the USA).

mileways: an obsolete term for an hour angle of 5º, equivalent to 20 minutes of time. So called because this is the approximate time that it takes to walk one mile.

minute of arc: see arc minute.

minute (of time): is now defined as 60 seconds. Historically, the definition was 1/60th hour, where the hour was derived from the rotational period of the Earth.

month: an interval of time related to one revolution of the Moon around the Earth (a "moonth"). The calendar month derives from the synodic month (full-moon to full-moon) which averages 29.53 days. The anomalistic month (perigee to perigee) averages 27.53 days.

Moon: the natural satellite of the Earth. It has a mean distance from the Earth of 384.4 x 103 km and a semi-diameter at mean distance of 15' 33". The inclination of its orbit to the ecliptic is 5º 8' 43". Note: "moon", without an initial capital letter, is sometimes used to refer to moons of planets other than the Earth.

moondial: see Dials (types of).

moonlight: rays of light which reach the observer directly from the Moon, having originally been sunlight reflected by the Moon's surface. There is usually sufficient light to cast a shadow only between the 1st and 3rd quarters of the Moon. Since the angular size of the Moon is approximately the same as that of the sun, the ratio of umbra to penumbra of a moon shadow is also the same as for a sun shadow.

motto: a sentence, phrase or verse inscribed on a dial expressing an appropriate sentiment. Mottoes started appearing on dials in the late 16th century but were particularly popular in the 19th century.