The Source of Summer's Sizzle

Summer is here--at least for us Northern Hemisphere humans. And that means it's time for beaches, barbecues, and a brief batch of summertime science from your friends at KnowledgeNews. Why does summer sizzle so? Our simmering minds want to know!

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, with 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 is not because we've cozied up to the celestial furnace. No, summer comes from the tilt of the Earth, which affects the intensity and duration of sunlight we get hit with throughout the year.

We Live in a Cockeyed Place

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 protoplanet 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 more or less points toward the same place in space throughout the year--at Polaris, the aptly named North Star.

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 your daily bake.

Half Baked by the Sun's Rays

The hemisphere leaning into the sun gets a whole lot of daylight every day, while its pasty counterpart gets less tanning time and more starry sky. In fact, because of the Earth's tilt, the north pole gets 24 hours of daylight every summer day, while the south pole sits the season 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 you. 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, it has to slug through far 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.