Today – June 20 – is the northern
Summer Solstice, sometimes known as the Northern Solstice, “first
day of summer”, or Midsummer's Day, depending on where you live.
It's the longest day and shortest night of the year in the northern
hemisphere (where I live), though exactly how
long or short depends on how far north you live. And of course
in the southern hemisphere, today is is the shortest day and longest
night, since the seasons are reversed.


Now we
can see why summers are hot! In summer, the Sun rises earlier, sets
later, and reaches a higher point in the sky. Those things combined
mean extra sunlight, heating up the air and the ground longer. We can
also see why I put “first day of summer” in quotes: the Solstice
is the apex of the process, but the increase in daylight and
temperatures begins long before June 20 (at least every place I've
lived). The Midsummer's Day festival, celebrated throughout northern
Europe, acknowledges that; Shakespeare's play A Midsummer
Night's Dream may have been
written for the English version of the festival (though from what I
can tell, the historical evidence is scant).
Similarly,
during winter the Sun's light comes in at a steeper angle and days
are shorter, so the time for the ground to warm is greatly reduced.
The northern Winter Solstice (also known as the Southern Solstice,
“first day of winter”, Midwinter's Day, or Yule) is the shortest
day and longest night of the year in the northern hemisphere. On that
day, the North Pole points as far away from the Sun as it ever does.
We also have the reason the tropics are warm all year around: they
receive about the same amount of sunlight during both summer and
winter.
Approximately
halfway between the solstices, the Sun appears directly overhead at
noon at the Equator. On those days, everywhere on Earth gets about 12
hours of daylight and 12 hours of night. These days are the
equinoxes, meaning
“equal night”. (The spell to extinguish light in the Harry Potter
books is “nox”, for what it's worth. Yes, I remember such things.
I'm still waiting for my “accio!” summoning spell, though.) The
two days are known as the Vernal
(or spring) and the Autumnal (or autumn) Equinox, again based on the
seasons in the northern hemisphere. From an astronomical point of
view, Earth's “solar year” is marked between successive vernal
equinoxes. (A second measurement of the year, known as the sidereal
year, is measured with respect
to the stars. These two year measurements are almost, but not quite,
the same length!)
Now
let's put all of this together in a movie! (For some reason, the Sun
– which was a gently glowing lamp in my original simulation –
came out looking flat and boring in the final movie. I guess I still
have more to learn about creating three-dimensional animations.) For best results, please view this full-screen.
You missed that far enough north (or south) the sun "never sets" during a part of the summer. And during winter, it equally never rise. That is above the Arctic circle and south of the Antarctic circle. That would be a good video to demonstrate to. :)
ReplyDeleteWell, I thought about including it, but decided to keep the length (relatively) short!
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