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A sundial wouldn't be very accurate at a magnetic pole during winter, on account of the lack of shadows. But apart from that, a sundial with a nice strong shadow is always accurate. It's important to note, however, that sundials measure "sun time" as opposed to "clock time." As we learned in Sundials on the Internet, the sine qua non of online sundial resources, "sun time" is predicated on the idea that when the sun reaches its zenith, it's noon. The time between successive noons varies throughout the year. Poking around some more in our Sundial Category, we learned the days lengthen and shorten throughout the year because
of two reasons: the Earth is tilted at an angle relative to the sun, and the Earth's orbit around the sun is elliptical. The North American Sundial Society states it simply: any sundial will be as much as 16 minutes fast or slow (relative to clock time) throughout the year. If you know your latitude and longitude, adjustments can be made for this, but it seems beside the point. Sundials simply measure the sun's trajectory across the sky.
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