出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2012/12/18 17:31:14」(JST)
Circular motion can occur in two possible directions. A clockwise (typically abbreviated as CW) motion is one that proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite sense of rotation or revolution is (in Commonwealth English) anticlockwise (ACW), or (in North American English) counterclockwise (CCW). In a mathematical sense, a circle defined parametrically in a positive Cartesian plane by the equations x = cos t and y = sin t is traced counterclockwise as t increases in value.
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Before clocks were commonplace, the terms "sunwise" and "deiseil" and even "doecil" from the Scottish Gaelic language and from the same root as the Latin "dexter" ("right") were used for clockwise. "Widdershins" or "withershins" (from Middle Low German "weddersinnes", "opposite course") was used for counterclockwise.[citation needed]
The terms clockwise and counterclockwise can only be applied to a rotational motion once a side of the rotational plane is specified, from which the rotation is observed. For example, the daily rotation of the Earth is counterclockwise when viewed from above the North Pole, and clockwise when viewed from above the South Pole.
Clocks traditionally follow this sense of rotation because of the clock's predecessor: the sundial. Clocks with hands were first built in the Northern Hemisphere (see main article), and they were made to work like sundials. In order for a horizontal sundial to work (in the Northern Hemisphere), it must be placed looking southward. Then, when the Sun moves in the sky (east to south to west), the shadow cast on the opposite side of the sundial moves with the same sense of rotation (west to north to east). This is why hours were drawn in sundials in that manner, and why modern clocks have their numbers set in the same way. Note, however, that on a vertical sundial (such as those placed on the walls of buildings), the shadow moves in the opposite direction, and some clocks were constructed to mimic this. The best-known surviving example is the astronomical clock in the Münster Cathedral, whose hands move counterclockwise.
Occasionally, clocks whose hands revolve counterclockwise are nowadays sold as an aid to the left-handed, or as a novelty. Historically, some Jewish clocks were built that way, for example in some synagogue towers in Europe. This was done in accordance with the right-to-left reading direction of the Hebrew language.[citation needed]
Typical nuts, screws, bolts, and bottle caps are tightened (moved away from the observer) clockwise and loosened (moved towards the observer) counterclockwise, in accordance with the right-hand rule.
A way of remembering this that is based on the right-hand rule is to place one's loosely-clenched right hand above the object with the thumb pointing in the direction one wants the screw, nut, bolt, or cap ultimately to move, and the curl of the fingers, from the palm to the tips, will indicate in which way one needs to turn the screw, nut, bolt or cap to achieve the desired result. Most threaded objects are susceptible to application of the above; for a countably small number of exceptions (read: "left-handed" threads, or "reverse threads"), one substitutes the left-hand rule instead.
The reason for the clockwise orientation of most screws and bolts is that supination of the arm, which is used by a right-handed person to turn a screw clockwise, is generally stronger than pronation. Also, it was wise to adopt a single standard version for most screws and bolts – in order to eliminate endless confusion.
Sometimes the opposite sense of threading is used for a special reason. A thread might need to be left-handed to prevent the prevalent stresses that are present from loosening it. For example, some older automobiles and trucks had right-handed lug nuts on the right side of the vehicle and left-handed lug nuts on the left side of the vehicle. As the vehicle moved forward, the lug nuts tend to tighten. For bicycle pedals, the one on the right must be reverse-threaded, or the pedal will unscrew during use; similarly, the flyer whorl of a spinning wheel uses a left-hand thread to keep from loosening in normal use. A turnbuckle has right-handed threads on one end and left-handed threads on the other end. Some gas fittings are left-handed to prevent disastrous misconnections. For example, oxygen fittings are right-handed, but acetylene and other flammable gases use left-handed fittings.
In trigonometry, and in mathematics in general, plane angles are conventionally measured counterclockwise, starting with 0° or 0 radians pointed directly to the right (or east). In navigation, compass headings increase in a clockwise direction around the compass face, starting with 0° at the top of the compass (the northerly direction).
Most left-handed humans prefer to draw circles clockwise and circulate in buildings clockwise, and most right-handed people prefer to draw circles counterclockwise and circulate in buildings counterclockwise. It is believed that this can be attributed to dominant brain hemispheres.[1]
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