January 4, 2012 - Earth at Perihelion

The Earth will be at its closest point to the Sun today at 7:00 p.m. U.S. Central Time. That closest-to-the-Sun point is called “perihelion.”

Most people think of the Earth circling the Sun in a circular orbit – see? I just used the verb circling? That's how we think of it! – but really its orbit is elliptical. That means it is slightly oval in shape.

(To be precise, a circle is a very special ellipse in which every point is equal distance from the geometric center, so what I should say is that the orbit of the Earth is a non-circular ellipse.)
This diagram shows last year's
perihelion (and also the point at
which Earth was farthest from the Sun).
NOTE: This diagram is WAY out of scale.

Every ellipse has two focus points. Try moving one of the foci of this ellipse, and see how the other focus changes as well to maintain the elliptical shape. How long and skinny an ellipse can you make? Can you make a perfect circle? What happens to the two foci in a circle?

  • Here is a website in which you can create an elliptical orbit for “your planet.” You can make your orbit closer to a circle or much more elongated. Notice, with an elongated orbit, that your planet goes a little faster when it is closer to the Sun and a little slower when it is farther away. Also try the zoom in/out and slower/faster sliders!

Also on this date:

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