Monday, July 4, 2011

Happy Aphelion

Yes, I know, everyone else celebrates the 4th of July.  Don't get me wrong, I'm a huge fan of the freedoms our Constitution and independence gave us, but there's something else that's notable today.

Today, July 4, at 1500 UTC (11:00 am EDT), the Earth will be at aphelion in its elliptical orbit around the Sun.  We're 152,097,700 km (94,509,130 miles) from the Sun today.  Perihelion, when the Earth is closest to the Sun (peri is Greek for "next to" and helios is the Greek word for the Sun), will occur on January 5, 2012 at 0100 UTC.  At that time, we'll be only 147,098,070 km (91,402,500 miles) from the Sun.  This is a difference of about 5 million km (3 million miles).

So, why is it so much hotter today than it will be in early January?  Because the difference in the Earth's position from the Sun between aphelion and perihelion is only about 3% and not really enough to have much of an effect on the amount of insolation (incoming solar radiation) reaching the Earth.  If it were the reason for the seasons, the Northern and Southern Hemispheres would both have summer in January and winter in July.

The real reason for the seasons is the 23.5° tilt of the Earth's axis.  The Northern Hemisphere is tilted toward the Sun on the June solstice giving us more direct sunlight and more hours of sunlight.  In the Southern Hemisphere, they're tilted away from the Sun so it's their winter  On the December solstice, we're tilted away from the Sun so we have less direct sunlight (it comes in at a lower angle) and less hours of sunlight.  The equinoxes, of course, are the half-way points between the solstices.

As we start to move closer and closer to the Sun over the next six months, the Earth will also begin to speed up in its orbital path - all part of Johannes Kepler's Three Laws of Planetary Motion - the subject of another post!

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