FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

Celebrating Earth's orbit

When, on the 4th of July, you are celebrating our nations's independence by watching fireworks, some of us might be celebrating for an additional reason. We are at aphelion! Let me explain what that is and why we should be celebrating.

All objects in our solar system - planets, minor planets, comets, asteroids, the whole kit and boodle - all travel around the sun, not in perfect circles, but in ellipses. What are those?

Ellipses are all over the place. They are often referred to as ovals. Here is an easy way to see a whole array of ellipses. Look at a cup from directly over the top. The rim is a circle. But as soon as you start to tilt the cup, that circle becomes an ellipse. Keep tilting it and the ellipse becomes more and more what we call "eccentric" until you get to the point where you finally look at the rim from edge on.

All those shapes were ellipses and just about every one of them can be seen in the orbits of objects in our solar system. But our planet's orbit, although elliptical, is almost a circle - but not quite.

There is one more very important thing to know about this phenomenon before we celebrate. The sun is not in the middle of any of these elliptical orbits; it is slightly off-center.

For the sake of simplicity, if not for accuracy, draw a simple circle then move its center slightly off to one direction. In your mind, picture a tiny sun there. That drawing is similar to how we travel round our off-centered sun.

Now here is the thing to notice. Something traveling around your drawn path will sometimes be closer to the "sun," sometimes farther. The closest we get to the sun in our orbit is called perihelion, literally "near the sun." The farthest point, on the opposite side, is called aphelion, literally "away from the sun."

Our planet reaches aphelion on the evening of the 4th of July. So why celebrate? Thinking cap time!

The weeks we are farthest from the sun also happen to be when we are tilted most towards it here in the northern hemisphere. Thus, we get the most amount of sunlight when we are farthest from its heat! That means our summers aren't nearly as hot - nor our winters as cold - as they could be. Reason for celebration right there.

But even more to be thankful for is that our aphelion distance next week is only several million miles farther out than our perihelion distance in January. On the big scale of our solar system, that means we orbit in almost a perfect circle.

If our orbit were more elliptical, more oval-like, our close approach to the sun would give us unbearably hot daytime temperatures, possibly lethal on a global scale. Our aphelion, just six months later, would be dangerously farther and Earth would be deadly cold.

So while celebrating our independence next week, take a moment and be grateful as well for our awesome orbit.
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me