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The Earth Reaches Perihelion

Week of January 4, 1999

Did last Sunday's sunset look a little bit bigger to you?

Probably not. But it was, just a wee amount.

In ancient Greece, Aristotle proposed that the Universe was perfect, and as such it could only behave in perfect ways. The perfect shape, according to him, was the circle, and since obviously the Earth did not move, the Sun (and hence the other planets too) must orbit the Earth in circles.

Around 140 A.D., the astronomer Ptolemy made a refinement on Aristotle's theory. He wanted to predict the positions of the planets and the Sun days or even months in advance, and found that a simple circular orbit for the Sun and planets around the Earth wasn't doing a good enough job. To correct for this, he proposed that each object, while still orbiting the Earth in a circle, also made little circles while going in the big circle. These little circles, called epicycles, did make predictions a little more accurate. But as measurements got better, it was found that one epicycle wasn't enough. So over the next thousand years astronomers added more epicycles to the orbits, making a diagram of the solar system an unwieldy mess.

Later, Copernicus made arguments that the Sun was the center of the solar system, and not the Earth. He was right, as we know today, but unfortunately he still thought the Earth orbited the Sun in a circle. He still needed epicycles to get things right, and his epicycles were just as bad as Ptolemy's!

Enter Kepler, in the late 1500s. A student of the great observer Tycho, Kepler made very careful observations of the planets. He found after studying Mars that the best fit to its orbit was not a circle at all, but an ellipse. Now, we may think this is obvious today, but back then it was a major discovery. It meant that sometimes the Earth was closer to the Sun than other times!

Which brings us back to Sunday's sunset. At 13:00 Universal (Greenwich) Time on Sunday, January 3, 1999, the Earth was at the point in its orbit when it was closest to the Sun (the technical term is perihelion). At that time, the Earth was 1.7% closer to the Sun than average. That means that to us, the Sun looked a tiny bit bigger, about the width of a human hair held at arm's length. That's why I doubt you noticed any difference!

Incidentally, in the northern hemisphere, the time of perihelion for the Earth is very close in time to the winter solstice. Some people think that seasons are caused by the distance of the Earth from the Sun, but hopefully readers of my Bad pages know better. In the southern hemisphere, perihelion is very close in time to the summer solstice. I wonder if that misconception is more prevalent Down Under?



©2008 Phil Plait. All Rights Reserved.

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