A Trip Around the Sun

(Updated because I am an idiot from time to time. It happens.)

Today is New Year's Eve, which means tomorrow is the start of a new year. But what is a year exactly? And would a year by any other name take as long?

While your life may not be ruled by Mercury in the 7th house, it is in fact defined by the stars. One year on the planet Earth is defined as the time it takes the Sun to complete its path through the zodiac along the ecliptic; in laymen's terms, one trip around the Sun. But in astronomical terms, a bit harder to define.

Let's start with the ecliptic. In the night sky, there are two important circles to keep track of. One is the celestial equator - take Earth's equator and project it out onto the night sky. That's it. It's essentially the equator of the stars. The other is the ecliptic - the plane of the Solar System in which all the planets (roughly) orbit the Sun. As the Earth is tilted on its axis 23 degrees, these two circles are also tilted with respect to each other, by the same amount. The zodiac is that familiar group of twelve constellations found along the ecliptic: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. Just as Earth's equator has an analogy in the sky, the celestial sphere also contains lines of longitude and latitude. Each sign of the zodiac spans 30 degrees of longitude, or about two time zones on the sky. The Sun appears to follow a path through the zodiac as Earth's travels in its orbit. A year would then seem to be when the Sun passed through all twelve constellations.

Not so fast.

Turns out, nothing about Earth's orbit is stable, it's all precessing. Like a spinning top that wobbles slightly, each of Earth's orbital elements is in flux, oscillating with different frequencies. Earth's axis, tilted 23 degrees with respect to the plane of the Solar System, precessing most like the spinning top, making one complete wobble every 26,000 years. In addition, the wobble has a wobble - the actual tilt of Earth's axis actually varies between 22.1 and 24.5 degrees on a larger 41,000 year cycle. Finally, the eccentricity of Earth's elliptical orbit around the Sun alternates between close to circular and moderately elliptical every 100,000 years. On top of this, the Moon's orbit precesses. All of the planets in the Solar System gravitational interact with each other, every so slightly tweaking orbits here and there. And the stars themselves aren't even fixed in place.

The combination of all these variables makes it pretty hard to determine when Earth has returned to an exact point in time and space. Astronomers have come up with a myriad of definitions for each and every cyclical alignment they can identify. There's the sidereal year and the tropical year and the anomalistic year, the draconic, eclipse or ecliptic year, the lunar year, the heliacal year, the sothic year, the gaussian year, the besselian year, and my personal favorite: the vague year. Each is defined with respect to Earth and specific celestial bodies and years ranging from 346 to 383 days. That's roughly a month-long interval of new years to celebrate.

Of course, for historical, political, and economic reasons, the calendar year we abide by is an approximation of Earth's orbital period. And other than the lunar year, the differences between these other years are imperceptible to you and me. But I see no reason why we shouldn't celebrate each and every one of them. It's too easy to forget to look up once in a while and ponder the bigger picture.

So let's get started. Happy new years to you and your's in all sense of the word and I hope you enjoy this next trip around the Sun, through the zodiac, with respect to another star, perihelion, and/or node here and there.

(Image credit: http://en.wikipedia.org/wiki/File:Ecliptic_path.jpg)