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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.





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Michael Lu 2015 年 12 月 29 日 に公開
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