Monday, January 5, 2009

Perihelion, Aphelion and Precession

This would have been the perfect entry for Sunday’s topic if only I had discovered it earlier. Yesterday the Earth was at Perihelion–its closest point to the Sun for the entire year. My husband asked why then it wasn’t warm. I knew the basic answer, but I did a little research to completely understand it myself and of course, the answer I found was a little more complex and utterly fascinating.

Our distance from the Sun doesn’t cause the seasons, the tilt of the Earth does. The Earth is tilted on its axis 23.5°. In January, Earth’s northern axis is tilted away from the Sun giving us cold weather. In early July, the northern axis will be tilted towards the Sun and we have warm weather.

Earth’s orbit around the Sun is not a perfect circle, but an ellipse shape. If you were to draw a horizontal ellipse, basically an oval, the Sun doesn’t sit in the middle, but off center, let’s say to the right for example. If the Earth were orbiting on the path of the ellipse, when it reached the right end, it would be the closest to the Sun, or Perihelion (3 million miles closer.) And when Earth reached the left end of the ellipse, it would be the farthest from the Sun, or Aphelion (1.8 million miles further.)

Here is where this somewhat simple explanation of the Earth’s tilt and orbit becomes more complex–Precession. The Earth not only rotates on its axis, it gyrates around its axis. For example, if you were to spin a top, it stays vertical until you give it a nudge or it starts to slow down and then the top leans to the side, gyrating around the vertical as it spins. The point of the top stays on the ground but the handle swings around above it, essentially tracing a cone shape.

We know the Earth rotates on its axis every 24 hours. But do you know how long it takes to complete its precession? 26,000 years. The cause of the precession is the equatorial bulge of the Earth as a result of the centrifugal force of the Earth’s rotation. The rotation changes the Earth from a perfect sphere to a slightly flattened one, thicker across the equator. The attraction of the Moon and Sun on the bulge is the “nudge” that is making the Earth precess.

Now this information just blows my mind. Greek astronomer Hipparchus of Nicea discovered the Earth’s precession in 130 BC! He compared his observations to those from the preceding 169 years and proposed that the axis around which the heavens seemed to rotate shifted gradually, though very slowly, and he concluded it had moved by 2 degrees. How Hipparchus of Nicea managed to discover Earth’s precession in an ancient 130 BC is astonishing to me. Yesterday, I couldn’t even figure out why the dryer wouldn’t start–it was a blown fuse.

Now you’re a little smarter, Girlfriend — And so am I.

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