Why do clocks run clockwise?

Clockwise is a convention, not a law of physics. The most common explanation is that clock hands imitate the shadow of a sundial in the northern hemisphere, where mechanical clocks were first developed. Sources describe this as reasonable, but one of them notes it is hard to establish with certainty, and a reader objection raises a problem with it. [1] [2]

In the northern hemisphere the Sun crosses the southern sky from east to west. A simple upright stick on a flat, horizontal surface therefore casts a shadow that sweeps around it in a clockwise direction, and the shadow's position marks the time. It has even been claimed that the word clockwise was chosen for this reason. At noon the shadow points north in northern middle latitudes. Its direction at other times of day can vary with the season, because the Sun's path is higher in summer than in winter. [3] [1]

In the southern hemisphere the Sun lies to the north, so a sundial shadow moves anticlockwise. The Irish Times notes that had mechanical timekeeping begun there, clocks might well run the other way. [4]

Shadow clocks and sundials were in use in Egypt and Babylon thousands of years ago. One source gives about 3,500 BC for the first shadow clocks, while another says most sources place the earliest true sundial at least 3,500 years ago in Egypt. Sundials stayed popular through the Middle Ages, and early mechanical clocks were often calibrated against nearby sundials. Gear-driven clocks appeared in Europe between northern Italy and southern Germany around 1270 to 1300. By the end of the 14th century, clock faces with hands were appearing, including on cathedral clocks, and one account says these imitated the sundial's clockwise motion. [5] [1]

Many early clocks had no hands at all and simply struck the hours on a bell. Hodinkee adds that mechanical clocks were first widely developed in the northern hemisphere, which helps explain why this direction became the global standard. [2]

The sundial account is the widely repeated one. Hodinkee's author says it is hard to establish with absolute certainty and that he found no specific historical document stating it. He points out that nothing in the mechanics favors clockwise motion, and still considers the idea reasonable. The Irish Times likewise says it seems reasonable to assume early clockmakers imported the familiar pattern. [2] [4]

A reader comment quoted on Today I Found Out raises an objection. Horizontal sundials in the northern hemisphere have clockwise shadows, but on vertical-faced sundials the shadow moves anticlockwise, and the first mechanical clocks probably had vertical faces. The results offer no reply to this point, so it remains an open question. [5]

Anticlockwise clocks are easy to build. They turn up as novelties in some pubs. In 2014 Bolivia's national congress building in La Paz received a clock whose hands move anticlockwise, part of an effort to challenge the northern-hemisphere standard. [4] [2]