A clockwise (typically abbreviated as CW) 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, continually turning right is clockwise motion, as viewed from above. The opposite sense of rotation or revolution is counterclockwise, the North American English term and the one used by the majority of the English-speaking world, and abbreviated CCW. Anticlockwise is the current British English term, and is perhaps used in much of the former British Empire (excluding Canada and the United States).

Origin of the term

Before clocks were commonplace, the terms "sunwise" and deiseil (from the Scottish Gaelic language from the same root as the Latin dexter, "right". This 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 counterclockwise.



Actually, the terms clockwise (abbreviated CW) and counterclockwise (CCW) 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 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. 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 Munster 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. This was done in accordance with the right-to-left reading direction of the Hebrew language.

Usage

Typical nuts, screws, bolt, 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 rough mnemonic for remembering this is "righty-tighty, lefty-loosey" (right to tighten, left to loosen). This mnemonic is ambiguous; depending on where the handle of the wrench, for example, is when the wrench is first applied to the nut (or bolt), moving it to the right may result in turning the nut (or bolt) clockwise or counterclockwise. Worse, when the wrench handle points exactly at the "three o'clock" (0°) or "nine o'clock" (180°) position, the mnemonic offers little help. Also, this mnemonic is applicable only to conventionally-threaded objects, those referred to as being 'right-handed' or as having 'right-hand' threads.

More generally, to the extent that this mnemonic can be applied at all, it only works when right and left are considered relative to an address of the top, or face, of the object, and not when the bottom, or back, of the object is being addressed. Analogously, the meaning of clockwise falls out when you are viewing the clock-face from within the clock - as you might on a tour of the Clock Tower, part of the Palace of Westminster, in London, England.

An alternative, simple-to-use approach - and one 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 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. 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. 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).

In humans

Most left-handed humans prefer to draw circles clockwise and circulate in buildings clockwise, since most right-handed people prefer to draw circles and circulate in buildings counterclockwise. It is believed that this can be attributed to a dominant brain hemispheres.

However, one can readily and consciously adopt the following rule at intersections: "When in doubt, turn right." This has two sources: A. In Latin, the word "sinister" means "left" and also "bad or evil", but the word "dexter" means "right" and also "good". B. In the majority of countries, such as in the Americas and on the mainland of Europe, where driving is done on the right-hand side of the road, it is really simpler to make right turns than left turns at intersections. Generalizing this to other situations causes wise people to explore unfamiliar buildings in clockwise paths regardless of the handedness of the person involved.

References

1. http://www.scrapbookpages.com/CzechRepublic/Prague/Josefov/JosefovHistory.html
2. Theodore H. Blau, The torque test: A measurement of cerebral dominance. 1974, American Psychological Association.

See also

• Handedness
• Chirality , Chirality
• Inner/Outer orientation
• Optical isomerism
• Retrograde motion
• Relative direction