FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

Proximity Matters in Planetary Real Estate

If you asked your friends what time of the year the Earth is closest to the sun, during winter or summer, I'd bet that most of them would say during summer. After all, we are hottest during the summertime, therefore we must be closer. Correct?

The reasoning seems airtight. But it presumes that our distance determines our seasons, and it neglects the fact that our southern hemispheric friends are experiencing a cold winter at the same time we're having our hot summer.

The fact is that for these last couple weeks we have been the farthest from the sun in our orbit. And it is in the dead of our winter that we are closest to the fires of our star. Huh?

As you know, every planet, comet, and space rock in our solar system orbits the sun. But despite the fact that planetary orbits are drawn as perfect circles in our textbooks, they are not.

But that's OK. We don't need a perfectly circular orbit. Slightly oblong - we call it "elliptical" in the Land of Science - is just fine. But here's the point: Since it is not a perfect circle, it follows there must be a time in an orbit when a planet is closer to the sun than at all other times, and another time when it is farthest.

And it is now, at the beginning of July, that we are at our farthest. This "farthest point" in an orbit around the sun has a name, aphelion. Our closest point to the sun happens in the first week of January. Then we are at perihelion.

Now a fair question is: Why aren't we freezing right now, being farther away? Well, the difference between our aphelion and our perihelion isn't that dramatic. Our orbit, although not a perfect circle, comes close. There is a difference of only a few million kilometers between the two. But we are so far out that a couple million klicks doesn't matter.

Here's an analogy: Suppose you are a hundred yards away from a bonfire. If you were to take a big step closer would you feel the temperature rise? How about a step farther away? Would you feel a lot colder? Probably not.

Same with the sun. Our tiny difference in aphelion and perihelion makes no traumatic change in climate over the year. It is our tilt that gives us our seasons. The important question is: Are we tilted towards the sun or away? We are tilted towards the sun now, hence we have summer.

Don't get me wrong! Any more elliptical, or an average distance any closer or farther from the sun means very bad times on Planet Earth. But as it stands now - from tilt to spin to distance - all is very, very good.

Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me