How does a sundial work?

The working principle of the sundial: During the day, the shadow cast by the objects illuminated by the sun is constantly changing, and the length of the shadow is also changing. The shadow is the longest in the morning, getting shorter as time goes by, and getting longer after noon; The direction of the shadow is changing. In the northern hemisphere, the shadow in the morning is in the west, the shadow at noon is in the north, and the shadow at night is in the east.

In principle, it can be timed according to the length or direction of the shadow, but it is more convenient to time according to the direction of the shadow. Therefore, the timing is usually based on the position of the shadow. Because the sundial must rely on sunlight, it can't be used on cloudy days and nights. Therefore, it is not enough to use the sundial to time, and other kinds of timers, such as water clocks, are needed to cooperate.

Extended data:

The sundial is timed by the direction of the sun's projection, but it will fail in rainy days and nights, and it can't be used during the vernal equinox and autumn equinox. Thus, the ancient craftsmen in China invented the leaky carving. As a timer, the use of cracks is more common and longer than sundials.

Leakage refers to a leaky pot for holding water, which is used to drain or hold water; Engraving refers to the division of a day's time unit, and the time of day and night is measured by the floating arrow of the leaky kettle.

When using the kettle-broken wine, water is injected into the kettle-broken wine, and the water flows out of the hole of the kettle-broken wine and into the container placed under the kettle. There is a mark engraved with time in the container, which is called an arrow. The arrow is supported by the ship and floats on the water. When water flows out or into the pot, the arrow shaft sinks or rises accordingly. The ancients could tell the exact time by looking at the mark on the shaft from the cover hole.

Baidu encyclopedia-sundial instrument

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