Nature & Environment, Science & Technology

Winter solstice


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Today, the northern hemisphere is experiencing the shortest day of the year. The winter solstice marks the shortest daylight period, but it’s not the day of the latest sunrise or earliest sunset. In the mid-latitudes the earliest sunset occurs in early December, while the latest sunrise is not until early January.

This misalignment occurs because of a discrepancy between “clock time” (which is based on 24 hours), and “solar time” (the time it takes for the sun to appear in the same position in the sky from one day to the next). In fact, it is 24 hours only four times a year, and never in December. It is at its shortest around 23 hours 59 minutes and 30 seconds, in early September, and at its longest around 24 hours 30 seconds in December.

The Earth’s axis is tilted 23.5 degrees, and the Earth’s speed varies because it moves in an elliptical orbit around the sun, accelerating when it is closer to the sun’s gravitational pull and decelerating when it is further away. The sun therefore in effect lags behind the clock for part of the year, then speeds ahead of it for another.

Sunrise

In December, these two factors combine in such a way that our days are actually a few seconds longer than 24 hours – as seen by the amount of time it takes to cross our local meridian (longitude) from one day to the next. In effect, this cumulative shifting pushes the time of solar noon several minutes later during December, advancing both sunrise and sunset times even as the days continue to shorten until December 21.

This also explains why the evenings draw in towards their earliest sunset a couple of weeks before the shortest day, and why the mornings continue to get darker until a couple of weeks after.

HT: Kris Griffiths

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