Farthest from the Sun

Tomorrow, as Americans throng outdoors for cookouts, they should remember to slather on the sunscreen and drink plenty of water. After all, it's summer here in the Northern Hemisphere, and the Sun will be beaming down intense light (including skin-damaging UV rays) and heat.

That may, for some, make it a little odd to hear that July 4th, the Earth will be further from the Sun than at any other time during the year. It's at the most distant point in its orbit, known as aphelion.

Follow up:

Earth's orbit, you see, is not quite circular (although it's close - the diagram above is not to scale in any respect). We're further from the Sun at some times (furthest around early July) and closer at other times (closest at perihelion in early January). Such small changes are not enough to make a significant difference in Earth's seasons, although it does make the Sun appear a tiny bit smaller in the sky. Just go outside on July 4th, and you'll see that being farther from the Sun isn't making it noticeably cooler around here...

The seasons on Earth are actually caused by the Earth's tilt; right now the Northern Hemisphere is tilted toward the Sun, giving us more direct sunlight, and thus our hot summer. For those south of the equator, the Southern Hemisphere is tilted away from the Sun, and the less direct sunlight means that it's winter down under. (This animation shows the relationship of Earth's orbit, the angle of sunlight, and the season.)

Now, if we were on Mars, the story would be a bit different. The primary cause of the seasons on Mars, as on Earth, is the tilt of Mars' axis. However, the orbit of Mars is less circular than Earth's orbit. This means that, unlike Earth, Mars is gets noticeably more sunlight when it's nearest the Sun than when it's furthest from the Sun. As a result, Mars has greater differences in its seasons compared with those of Earth. And poor Pluto - its orbit is so non-circular that when it's far out from the Sun, it gets so cold that much of its atmosphere may actually freeze solid!

Hat tip: spaceweather.com