Occasionally, clocks whose hands revolve counterclockwise are sold as a novelty. The best-known surviving example is the Münster astronomical clock, whose hands move counterclockwise. Some clocks were constructed to mimic this. This effect is caused by the plane of the dial having been rotated through the plane of the motion of the sun and thus the shadow is observed from the other side of the dial's plane and is observed as moving in the opposite direction. For a vertical sundial (such as those placed on the walls of buildings, the dial being below the post), the movement of the sun is from right to top to left, and, accordingly, the shadow moves from left to down to right, i.e., counterclockwise. This is why hours must be drawn in horizontal sundials in that manner, and why modern clocks have their numbers set in the same way, and their hands moving accordingly. Then, when the Sun moves in the sky (from east to south to west), the shadow, which is cast on the sundial in the opposite direction, moves with the same sense of rotation (from west to north to east). In order for such a sundial to work north of the equator during spring and summer, and north of the Tropic of Cancer the whole year, the noon-mark of the dial must be placed northward of the pole casting the shadow.
Clocks with hands were first built in the Northern Hemisphere (see Clock), and they were made to work like horizontal sundials. The shadow of a horizontal sundial in the Northern Hemisphere rotates clockwiseĬlocks traditionally follow this sense of rotation because of the clock's predecessor: the sundial.
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