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Sunrise still later after Winter Solstice

Several people asked me about the odd phenomenon that in these here parts sunrise gets later each day until early January even though the days themselves get longer after the winter solstice.

sunrise

From EarthSky: The winter solstice always brings the shortest day to the Northern Hemisphere and the longest day to the Southern Hemisphere. But, the tardiest sunrise doesn’t coincide with the day on which the sun is above the horizon for the shortest time, least daylight hours; similarly, the latest sunsets don’t happen on the day of greatest daylight.

Why is this? The main reason is that the Earth’s rotational axis is tilted to the plane of our orbit around the sun. If it were perpendicular to the orbital plane we wouldn’t perceive this discrepancy.

A secondary reason is that the Earth’s orbit is eccentric (an ellipse, like a squashed circle, with the centre of the sun slightly off its centre), Earth travels fastest in January and slowest in July. Clock time gets a bit out of sync with sun time – by about 30 seconds each day for several weeks around the winter solstice. Adapted from Latest sunrises for mid-northern latitudes in early January.

Additionally, says Royal Museums Greenwich, the longest natural day is about 51 seconds longer than the shortest. But, for clocks to be useful, days need to be fixed in length. We fix them on the average, or mean, length of a natural day (hence Greenwich Mean Time). By averaging out the length of each day like this, the clock time at which the sun reaches its highest point slowly drifts back and forth as the months progress. There is a knock-on effect on the times of sunrise and sunset. The earliest sunrise occurs a number of days before the longest day and the latest a number of days after the shortest.

Originally posted 3rd January 2014

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