Happy Perihelion Day!

Today is Perihelion Day, the day when the Earth makes it`s closest approach to the Sun. Many people find it hard to believe that a frosty winter eve immediately following the New Year could host such an event, but the seasons are ruled by the slant of light striking the Earth, not by the distance the Earth is from the Sun. In summer, our part of the Earth (for those of us in the Northern Hemisphere) is tilted toward the Sun, so the rays of light strike us dead on, while the days are longer. In winter this situation is reversed, with sunlight coming at an angle and long winter nights. The Earth`s wobble is approximately 23*, and it is this wobble which gives us the seasons. This year, Aphelion Day (the day the Earth is farthest from Sol) will be July 7.

The Earth and planets move in elliptical orbits, as opposed to circles. This concept was discovered by Johanne Kepler, who was able to explain the bizarre tendency of the planets to seemingly move backwards in their orbits. These epicycles had confounded astronomers for centuries, and the establishment of Copernicus` heliocentric theory did not resolve the problem; under Copernicus it was still necessary to postulate a reversal of motion, although there were less of them. It`s little wonder that the motion of the planets was poorly understood, given this preposterous reality! Once Kepler realized that the planets moved in an ellipse, he was able to explain that the epicycles were merely illusions.

So all of the planets-including the Earth-move closer to and away from the Sun. Some objects (asteroids and comets, and even Pluto to a small degree) have orbits which are more elongated. These are said to be eccentric. Comets have some of the most eccentric orbits of all, dropping from the Oort Cloud (perhaps as much as half a light year) to dip as close to the Sun as Mercury. Fortunately, our orbit is much more stable, and we really don`t have to worry about the effects of orbital eccentricity.

So, enjoy your Perihelion Day; take the day off, drink a few brews, maybe even throw a Perihelion party with a few dozen of your closest friends! Remember that Perihelion only comes once a year!