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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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