Hot Time, Summer in the City
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Ask Americans why we're hotter in the summer than in winter and most will answer it's because that's when we are closest to the sun. Nothing could be farther from the truth.
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Earth (at apehelion) and the other planets on the Fourth of July.
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Just last week we celebrated the solstice, the longest day of our year, the beginning of summer in the Northern Hemisphere. Just a couple weeks later, on the Fourth of July, Earth will be at aphelion. Aphelion is fancy astrospeak meaning "farthest from the sun." It's strange but true: at the beginning of summer we are farthest away from that fireball!
Like all planets, Earth travels not in a circular orbit, but in one more oval-shaped. All the planets and asteroids and comets travel in these elliptical orbits, some more severely than others. Regular readers here will recall that in such an orbit there is a close point to the sun and a far point. Mars, for example, is now near its close point, perihelion, while we on Earth are just over a week away from our farthest, aphelion.
But if we are so far away now, why isn't summer cooler?
Our aphelion isn't too far away. We average about 93 million miles from the sun. On the 4th, we will be 94.5 million miles away. If you were a hundred meters from a bonfire and took a step farther away, it wouldn't get too much colder.
So
what causes summer to be so hot?
It's the Earth's tilt. During this time in our orbit, in June and July, we in the Northern Hemisphere are tilted most toward the sun. This means we have days that are longer, and a sun that rises higher in the sky.
A day which sees a lot of light, and in which that light strikes rather directly from on high adds up to a load of heat energy here on the surface. That's primarily why our summer is hot.
Careful listeners in world geography class will remember that our friends down under in Australia and Southern Africa and South America experience winter just as we sweat our way through summer. They all are currently tilted away. Their sun is up for much less of the day and never rises too high above the horizon to get things really hot and bothersome.
Here it is in the Big Perspective.
Our
orbit is not quite circular, but it nearly is. This allows our aphelion
and perihelion to be no immediate danger to us. But, if our orbit were
any more elliptical, our perihelion would fry us and our aphelion would
freeze us -- annually.
As to our tilt: If we were tilted over much more than we are now our summers would get toasty beyond all comfort and our winters will be numbingly frigid. No tilt would mean the equatorial regions would be considerably warmer and the polar ice caps would be enormous. The only hospitable place on our planet would be a very narrow band between the pole and the equator.
The shape of our orbit and our tilt are not too much, not too little ... why, they're just right for a maximum-comfort planet.
So, come this Fourth of July, before the fireworks go up, go out and look at the big Work of Fire in the sky and be thankful that it's exactly where it should be.
I have images to help you visualize all this at firstlightastro.com.
Until next time, clear skies and Happy Aphelion!
Quicktime Movie of sun crossing whole sky on Summer Solstice (1.3MB)
Quicktime Movie of sun crossing whole sky on Winter Solstice (888K)
Posted by Administrator at 2001.06.23 08:23 AM | Comments (0)
