On what principle does sundial work? What is a drawback of a sundial?

The sundial works on the principle that when the sun is at highest point in the sky the image of an object formed by its light is smallest and when it is at the lowest point the image formed is the longest.
  • The most major drawback of sundial is that it cannot be used in the night.
  • Further it has to be kept outside  and cannot be used inside the house or it cannot be wore on a wrist.
  • Also, the length of the shadow formed by sunlight is effected by seasons as well and hence it cannot be used to predict the correct time in different seasons.

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Sundials rely on the fact that, as the earth turns, the sun seems to travel across the sky. Any object standing in the sun casts a shadow. On a stationary object, this shadow will move "clockwise" (in the northern hemisphere). This means that the relative times of the day can be marked around the stationary object, and during daylight hours an approximation of the actual time can be made.

The only point at which true sun time can easily be determined is at 12 noon; when, in the northern hemisphere, the shadow cast by the gnomon (the vertical part of the sundial) will be at its most northerly. If the Earth were not tilted as it orbits, then the sundial would be regular, and it would be quite easy to tell time by a sundial once the position of the hours around the sundial were established. However, the fact that the number of daylight hours waxes and wanes over the course of the year, and the fact that the angle any particular part of the planet will be, relative to the sun, changes on a daily basis means that the shadow cast by a sundial perpendicular to a tangent of the Earth will not be as regular as a clock. (This also stems from the fact that the Earth's orbit is not exactly circular, but is slightly elliptical.)
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