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  • In the Northern Hemisphere, December has the fewest hours of daylight and the most darkness

  • because at that time, the tilt of the Earth's axis is pointing the Northern Hemisphere away from the Sun.

  • However, as counterintuitive as it might sound,

  • December actually has the longest days of the year.

  • Modern clocks, of course, think everyday is 86400 seconds long,

  • but that's just the average length of a solar day of the course of the year.

  • A solar day is when you actually measure with a sundial

  • or equivalently it's the time it takes for a line of a longitude of the Earth to rotate back to face the Sun again.

  • This actually requires slightly more than 360 degrees of rotation because the Earth isn't just sitting in space rotating.

  • It's also moving around the Sun, so it has to rotate roughly 361 degrees

  • before the sun comes back perfectly overhead a particular place on consecutive days.

  • If the Earth's orbit were perfectly circular and its axis were perfectly upright,

  • that would be the end of the story.

  • However, the Earth's orbit is elliptical, so sometimes the Earth is slightly closer to the Sun,

  • and the peculiarities of gravity mean it moves faster when it's closer to the Sun,

  • so it goes farther around the Sun in 24 hours,

  • so the Earth has to rotate slightly farther before the Sun comes back right overhead.

  • 0.033 degrees farther, to be precise.

  • More rotation takes more time,

  • so when the Earth is closest to the Sun, the real sundial measured day length is lengthened by about 8 seconds.

  • Plus, the Earth's axis is tilted which is what gives rise to the seasons,

  • but also means at the time of the year, when the tilt points toward or away from the sun ,

  • narrower slices of longitude are aimed directly at the sun,

  • so as the Earth moves in its orbit,

  • it has to rotate slightly farther in order for a particular line of longitude

  • to catch up with the changing directions of the sun.

  • 0.088 degrees farther to be precise.

  • And again, more rotation takes more time,

  • so when the Earth is tilted towards the sun,

  • the real sundial day length is lengthened by about 21 seconds.

  • Now, by a strange coincidence,

  • we live during a time in geological history

  • when the Earth's closest approach to the sun happens almost perfectly to coincide

  • with one of the two times of the year when the Earth's tilt is oriented directly towards the Sun.

  • So these two day-lengthening effects add up

  • and on December 22nd, the length of a solar day as measured by a sundial

  • will be the longest day all year!

  • 86430 seconds, for a grand total of 30 extra seconds.

  • Oh, and in case you're wondering where those extra 30 seconds on December 22nd go

  • Well, they get pushed into December 23rd,

  • and the extra seconds from the 23rd pushed into the 24th and so on

  • which is why solar noon or the time that the sun is directly overhead

  • shifts about 30 seconds everyday around the solstice.

  • This increasing disparity between solar time and clock time

  • is also why in the Northern Hemisphere

  • the earliest sunset happens a few weeks before the solstice

  • and the latest sunrise happens a few weeks after.

In the Northern Hemisphere, December has the fewest hours of daylight and the most darkness

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Why December Has The Longest Days

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    Michael Lu posted on 2015/12/28
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