For Northern Hemisphere humans, summer is officially here. And so, unfortunately, is summer heat. Of course, you already know why summer is hotter than winter, right? Well, be careful. Common wisdom isn't quite so wise here. For some truly trusty summer science, let's look directly at the sun.
Why Does Summer Simmer So?
The basic player in summer heat is, you guessed it, the sun. It must be a lot closer to us in the summer than in the winter, right? Not true.
Actually, because of the Earth's elliptical orbit, we're closest to the sun in January and farthest from it in July. So the reason we get scorched by summer heat is not because we've cozied up to the celestial furnace. The real reason lies in the tilt of the Earth itself, which affects the intensity and duration of sunlight we get hit with throughout the year.
Astronomically, Earth's a little off-kilter, rotating on its axis at an incline, or tilt, of 23.5 degrees. Why the La-Z-Boy position? Scientists think that sometime early on, Earth got absolutely clobbered by a Mars-sized proto-planet in a spectacular collision of worlds. That collision knocked Earth into a tilted rotational axis. Earth stays in this 23.5-degree tilt no matter where it is in its annual orbit around the sun. In fact, the northern end of the Earth's tilted axis points toward the same place in space throughout the year--right at Polaris, the aptly named North Star.
Summer solstice. Sun directly overhead at the Tropic of Cancer (23.5° N latitude).
Winter solstice. Sun directly overhead at the Tropic of Capricorn (23.5° S latitude).
Because of this tilt, Earth's north pole leans 23.5 degrees toward the sun on the summer solstice (around June 21), while the south pole leans 23.5 degrees away. On the winter solstice (around December 21), the south pole leans toward the sun and the north pole leans away. Whichever hemisphere leans toward the sun gets pool parties and picnics. The other hemisphere hauls out parkas.
But let's be clear here: the pool-party hemisphere doesn't heat up because it's closer to the sun. It heats up because that hemisphere receives the sun's solar energy at a more direct angle, which affects both the duration and intensity of its daily bake.
The hemisphere leaning into the sun is like an early season sunbather. It gets a lot of tanning time every day, while its pasty counterpart gets far less blazing daylight and a whole lot more starry sky. In fact, because of the Earth's tilt, the north pole gets 24 hours of daylight on the summer solstice, while the south pole sits the whole day out in darkness. (The equator, on the other hand, is Earth's Even Steven, with 12 hours of daylight every day of the year.)
You get a more intense bake in the summer, too. Think of summer as a sort of seasonal high noon. Like the hot, directly overhead, noontime sun, the summer sun points right at us--and that means more intense solar energy. It's the same if you angle your reading lamp at the wall. Not much heat in that oval patch of light, is there? Now point it straight at the wall. A bit warmer, even if you pull the lamp back a bit.
With the sun more directly overhead--like at noontime, in summer, or at the equator--sunlight effectively tunnels straight into our air. At the poles, during the morning or evening, or in the winter, however, it's got to slug its way through dramatically more atmosphere, as much as 40 times more. Sure, it gets to the same place eventually, but at the expense of a lot of energy.
Bottom line: Summer happens because Earth is a cockeyed place. No tilt, no seasons. But knock the world a little off-center, and for at least three months a year, you get a lot of light aimed right at you and concentrated through a thin atmosphere. You'll want to stay poolside.
Want to learn more?
Learn more about the summer sun with NASA scientists
http://helios.gsfc.nasa.gov/sun.html
From: KnowledgeNews
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