The sundial is very impressive, but it can only give you local solar time – which can be up to 16.5 minutes different from “watch time.” This difference is called the equation of time and is primarily due to the shape of the Earth’s orbit around the sun. When the Earth is closer to the Sun, it moves faster in its orbit so the length of the solar day changes fastest then – about 27-28 seconds in late December. In June, the daily change is very small. So the sun is always “fast” or “slow.” There are four days of the year when the equation of time is zero and the sundial is very close to correct: in 2017 these days were April 15, June 13, September 2 and December 25. Due to leap years, this can change by a day or so.
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