Sunset gets later before winter’s shortest day: the time paradox explained

Weather & Traffic In-Depth

AUSTIN (KXAN) — We all know that the Winter Solstice typically falls on December 21st, including this year, and it’s the shortest day of the year in terms of daylight. In Austin, on that day we only receive roughly 10hrs and 11 minutes of daylight.

With that said, our shortest day of the year is not when we experience our earliest sunset, nor is it when we experience our latest sunrise. If you look at the sunrise and sunset times for any city in the northern hemisphere around this time you will notice that the earliest sunset of the year occurs a few weeks before the Solstice and the latest sunrise of the year happens a few weeks after the Solstice.

In Austin, the earliest sunset is at 5:30 P.M. We have already started to experience that since the last week of November with our sunsets already starting to get later (since December 10th) even though the Solstice has yet to occur. By the Winter Solstice our sunset will occur at 5:35 P.M. Now despite a later sunset, the day still gets shorter up until the Winter Solstice. This is because sunrise occurs later and later — at a faster rate than the sun sets.

So why does all this weirdness happen? A discrepancy between modern-day timekeeping and the true “solar time.” Using a clock, humans define a day as 24 hours. But technically, this doesn’t match up to full day using solar time. Where a “full day” is the duration from one solar noon to the next, when the sun is at its highest point in the sky.

Our solar noon for today, for example, is 12:27 P.M!

So what gives? For starters, the earth’s axis is tilted by 23.5 degrees. It also has an elliptical orbit around the sun, and the sun isn’t even at the center of that orbit. These three things combined mean our day isn’t exactly 24 hours and it’s the reason our earliest sunset is in early December and our latest sunrise is in January — weeks after the shortest day of the year.

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