Clockwise
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A clockwise motion is one that proceeds 'like the clock's hands': from the top to the right, then down and then to the left, and back to the top. In a mathematical sense, a circle defined parametrically in a positive Cartesian plane by the equations x = sin t and y = cos t is traced clockwise as t increases in value. Described another way, constantly turning right is clockwise motion, as viewed from above. The opposite sense of rotation is anticlockwise (the current British English term), or counterclockwise (now chiefly North American English).
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[edit] Origin of the term
Before clocks were commonplace, the terms 'sunwise' and deiseil (from the Scottish Gaelic from the same root as the Latin dexter, "right". The word is also used for "ready") were used for clockwise. (Of course, deasil (righthandwards) is only sunwise in the Northern Hemisphere.) 'Widdershins' or 'withershins' (from Middle Low German weddersinnes, "opposite course") was used for anticlockwise.
Technically, the terms clockwise and anticlockwise 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 anticlockwise when viewed from the North Pole, and clockwise when viewed from 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). That's why hours were drawn in sundials in that manner, and that's why modern clocks have their numbers set in the same way.
Occasionally, clocks whose hands revolve anticlockwise are nowadays sold 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 Hebrew [1].
[edit] Usage
Typically, screws, bolts, and bottle caps are loosened (moved towards the observer) anticlockwise and tightened (moved away from the observer) clockwise, in accordance with the right-hand rule. One mnemonic for remembering this is "righty-tighty, lefty-loosey" (right to tighten, left to loosen.) (R H Daniel, 1980) The problem with the mnemonic is that it only works when viewing right and left relative to the top of the circle. When viewing relative to the bottom, the rule becomes invalid. Another simple to use procedure based on the right-hand rule, is to point the thumb of the right hand for right-handed threads or left hand for left-handed threads in the direction one wants the screw, nut or bolt to move, then the fingers of the hand will curl in the direction one needs to turn the screw, nut or bolt to achieve the desired result.
The reason for the clockwise orientation of most screws is that supination of the arm, which is used by a right-handed person to turn a screw clockwise, is generally stronger than pronation.
Sometimes the opposite sense of thread is used for a special reason; a thread might need to be left-handed to prevent the prevalent stresses from loosening it. In a pair of bicycle pedals, for instance, one must be reverse-threaded, or the pedal will fall off; similarly, the flyer whorl of a spinning wheel uses a left-hand thread to keep from loosening in normal use. Some gas fittings are left-handed to prevent disastrous misconnection; for example, oxygen fittings are right-handed but acetylene and other flammable gases use left-handed fittings.
In trigonometry, and mathematics in general, plane angles are conventionally measured anticlockwise. In navigation, compass headings increase in a clockwise direction around the compass card, starting with 0° at the top of the card.
[edit] In humans
Most left-handed humans prefer to draw circles clockwise and traverse buildings clockwise, as most right-handed people prefer to draw circles and traverse buildings counterclockwise. It is believed that this can be attributed to a dominant brain hemispheres. [2]
[edit] References
- ^ http://www.scrapbookpages.com/CzechRepublic/Prague/Josefov/JosefovHistory.html.
- ^ Theodore H. Blau, The torque test: A measurement of cerebral dominance. 1974, American Psychological Association.
