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Until the invention of mechanical clocks in the 14th century, sundials were the best way of telling time. Even long after they were replaced by mechanical timepieces, sundials remain in use to this day as functional decorations to homes, gardens, and parks. A sundial is a device that measures time by the position of the sun. It does this because the sun appears to move through the sky. At different times in the day when the sun is shining, a shadow is cast in different places on the dial, this lets users easily see the time. There are a few commonly seen designs, such as the 'ordinary' or standard garden sundial. However, sundials can be designed for any surface where a fixed object casts a predictable shadow. To set up a sundial requires a fair bit of knowledge about longitude, latitude and how to find the true north. Using a compass won't help as this will point to the magnetic north or south pole. Also adjustments have to be made in regions with daylight saving time. In terms of longitude, most sundials need to be set up so they're exactly parallel with the axis of the planet to function properly. For example, people in the Northern Hemisphere need to find the north pole. Latitude also poses a challenge if a sundial is set up at a different latitude than the one it was created for. Chances are good it will need to be carefully angled to function properly.
In common designs such as the horizontal sundial, the sun casts a shadow from its style (a thin rod or a sharp, straight edge) onto a flat surface marked with lines indicating the hours of the day. As the sun moves across the sky, the shadow-edge progressively aligns with different hour-lines on the plate. Such designs rely on the style being aligned with the axis of the earth's rotation. Hence, if such a sundial is to tell the correct time, the style must point towards true north (not the north or south magnetic pole) and the style's angle with horizontal must equal the sundial's geographical latitude.
The chief advantages of the horizontal sundial are that it is easy to read, and the sun lights the face throughout the year. A sundial designed for one latitude can be used in another latitude, provided that the sundial is tilted upwards or downwards by an angle equal to the difference in latitude. For example, a sundial designed for a latitude of 40° can be used at a latitude of 45°, if the sundial's plane is tilted upwards by 5°, thus aligning the style with the earth's rotational axis.
Images on this page courtesy of Artist David Harber
Horizontal garden sundial
Armillary Sphere garden sundial