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.
Origin of the term
Before clocks were commonplace, the terms "sunwise" and "deasil", "deiseil" and even "deocil" 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.
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 clockwise when viewed from above the South Pole, and counterclockwise when viewed from above the North 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 a novelty. Historically, some Jewish clocks were built that way, for example in some synagogue towers in Europe, to accord with right-to-left reading in the Hebrew language. In 2014 under Bolivian president Evo Morales, the clock outside the Legislative Assembly in Plaza Murillo, La Paz, was shifted to counterclockwise motion to promote indigenous values.
Typical nuts, screws, bolts, bottle caps, and jar lids are tightened (moved away from the observer) clockwise and loosened (moved towards the observer) counterclockwise, in accordance with the right-hand rule.
To apply the right-hand rule, 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. Almost all threaded objects obey this rule except for a few left-handed exceptions described below.
The reason for the clockwise standard for most screws and bolts is that supination of the arm, which is used by a right-handed person to tighten a screw clockwise, is generally stronger than pronation used to loosen.
Sometimes the opposite (left-handed, counterclockwise, reverse) sense of threading is used for a special reason. A thread might need to be left-handed to prevent operational stresses from loosening it. For example, some older automobiles and trucks had right-handed lug nuts on the right wheels and left-handed ones on the left wheels, so that, as the vehicle moved forward, the lug nuts tended to tighten rather than loosen. For bicycle pedals, the one on the left must be reverse-threaded to prevent it unscrewing during use. Similarly, the flyer whorl of a spinning wheel uses a left-hand thread to keep it from loosening. A turnbuckle has right-handed threads on one end and left-handed threads on the other. Some gas fittings are left-handed to prevent disastrous misconnections: oxygen fittings are right-handed, but acetylene, propane, and other flammable gases are unmistakably distinguished by left-handed fittings.
In trigonometry and mathematics in general, plane angles are conventionally measured counterclockwise, starting with 0° or 0 radians pointing directly to the right (or east), and 90° pointing straight up (or north). However, in navigation, compass headings increase clockwise around the compass face, starting with 0° at the top of the compass (the northerly direction), with 90° to the right (east).
Most left-handed people 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. This is believed to result from dominant brain hemispheres, though some attribute it to muscle mechanics.
- Chirality (physics), Chirality (chemistry)
- Inner/outer orientation
- Optical isomerism
- Retrograde motion
- Relative direction
- Sam Jones and Sara Shahriari (June 25, 2014). "Bolivia turns back the clock in bid to rediscover identity and 'southernness'". The Guardian. Retrieved June 26, 2014.
- Theodore H. Blau, The torque test: A measurement of cerebral dominance. 1974, American Psychological Association.
- Ack Demarest & Lorrie Demarest, 'DOES THE TORQUE TEST' MEASURE CEREBRAL DOMINANCE IN ADULTS?', 1980, Perceptual and Motor Skills: Volume 50, Issue , pp. 155-158 http://www.amsciepub.com/doi/abs/10.2466/pms.19126.96.36.199?journalCode=pms