Tuesday, January 27, 2009

The Ecliptic

The plane of the Earth's orbit around the Sun is called the ecliptic. It is the path the Sun appears to follow against the background stars. Both the Sun and moon must be in this band for eclipses to occur. It also forms the part of the sky in which the planets are always seen.

In other words, as the Earth orbits, the inside of the orbit always faces the Sun, while the outside faces a band of stars which changes predictably each year.

Ancient astronomers were able to trace the ecliptic as a line across the sky, even though they didn't realize that the Earth revolved around the Sun. They simply mapped the position of the Sun relative to the other stars in the sky, noticing that every 365 days or so the constellations would recur, and the Sun would start going over the same locations again.

An equinox is a time of year when our planet is at a location where the ecliptic plane intersects the equator. On the day of the equinox, there are as many minutes of daylight as there are of night, hence the term "equinox," meaning "equal darkness".

We all know that the Earth spins like a top. Like a top, it wobbles a bit while it spins, so that its axis traces out a circle in the celestial sphere. This wobbling is called "precession". The effects of precession are barely noticeable to the human eye, so ancient astronomers must have kept records for thousands of years to have realized that it was occuring.

The ecliptic is divided into twelve sectors, named for the constellations they contain, commonly known as the signs of the Zodiac. Because of precession, the dates when the Sun is in each constellation no longer lines up with the dates associated with the astrological house of the same name.

Additionally, we have divived the year into equally long sectors even though the constellations aren't all the same size. The Sun passes through Scorpio in only seven days, while Virgo takes 44 days.

The appearance of each constellation of the Zodiac occurs predictably, but changes very slightly each year. It takes 2150 years for the equinox to shift from one constellation to the next, called an astronomical age. We are currently in the Age of Pisces (the Sun is in Pisces during the spring equinox), but in 2150 will enter the Age of Aquarius.
One complete wobble of the Earth takes 25,800 years, called a Great or Platonic Year.

More on precession in my next astronomy post. For my previos posts on astronomy, click here.

For an awesome coffee table book on outer space, filled with amazing pictures from all over the Universe, check out Astronomy: A Visual Guide, by Mark A. Garlick.


Hans-J. Fuchs said...

Very nice article! What do we see in the awesome picture lighthouse.jpg?
Is it just some fantasy or is it an overlay of some astronomical events, separated by some time intervals?

Hansisgreat said...

The lovely photo was taken using time-lapse photography with a very slow exposure speed. In the foreground is the picturesque Fire Island Lighthouse, built in 1826 and still in operation today.