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Sun Time

December 3, 2017

The Sun isn’t a very good clock. The time it takes to move from one “noon” to the next can vary by as much as half a minute per day. That doesn’t have much impact on our daily lives. But it does create some interesting effects. One example is that the year’s earliest sunset comes several days before the shortest day of the year.

The solar clock varies for several reasons. The main one is that Earth’s orbit around the Sun isn’t a perfect circle, so our distance from the Sun varies by a few million miles. And that means that our orbital speed varies, too.

Right now, we’re fairly close to the Sun, so we’re moving faster than average in our orbit. So as Earth turns on its axis, it catches up to the “noontime” Sun a little earlier each day — up to about 30 seconds. Over a period of days or weeks, that can add up to several minutes.

Because of that, sunrise and sunset occur a little earlier, too. In fact, for those at the latitudes of the United States, the earliest sunsets of the year come a few days or weeks before the shortest day of the year — the winter solstice, which is coming up on December 21st.

The exact date of earliest sunset varies by latitude. For those in the south, it happens about now. From the latitude of Denver or Philadelphia, it happens in a few days. And from Minneapolis or Seattle, it comes closer to the solstice.

The date of the latest sunrise comes about the same gap after the solstice — the result of the imperfect solar clock.


Script by Damond Benningfield

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