Why does my sundial tell a different time than my watch does?
Sundials find the time at each instant by measuring the position of the sun in the sky. Sundials respond to Nature and measure natural cycles such as the earth's revolution around the sun. If the natural motions are not uniform, the time on the sundial will also not be uniform with your clock.

Example: We have seasons because of the orbit of the earth around the sun and the angle at which the earth is tilted on its axis. One consequence is that days are longer (there are more daylight hours) in the summer than in the winter. Sundials find more sunny hours in the summer than the winter because they show that the sun is above the horizon longer in the summer than in the winter.

Clocks measure how much time has elapsed since the moment at which their hour was set. Clocks respond to man-made mechanisms such as watch springs and vibrating quartz crystals. These mechanisms tick off uniform time beats, which add up to 24 uniform hours. Clocks do not respond to seasonal changes like the number of sunny hours. Clocks measure "mean time," which is the average time given by a machine rather than the time according to Nature. Sundials find the local solar time (the time at a specific place on the earth), whereas clocks keep the mean time (the average time assigned to the Time Zone, which is a broad 15 degree swath of the earth).

The sundial finds the local solar time at each instant, just as a thermometer finds the local temperature at a given moment. The clock gives the average time for a large region, just as the weather forecast gives the average temperatures for a region.

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