Autumn arrives at 6 p.m. Saturday, give or take a minute or two. We know that the temperature won't drop and the leaves won't change overnight just because the calendar says "first day of fall," but the date does mark a kind of mental shift as we prepare ourselves for the coming seasonal change.
Astronomers can mathematically calculate the exact moment of the Autumnal Equinox, the official name for Saturday's big event. Two times a year, once in the spring (the Vernal Equinox) and once in the fall, our tilted Earth is pointed neither toward nor away from the sun. The length of day and night is the same, the "equal night" that produced our word "equinox."
The other two major orbital events are the Summer and Winter solstices, when the Earth's Northern Hemisphere is pointed directly toward the sun (summer) or directly away from the sun (winter).
Seasonal shift: The tilt, of course, is the reason for the seasons, not Earth's distance from the sun. Earth's yearly trek around the sun isn't a perfect circle. Sometimes we're as far away as 94 million miles; sometimes we're as close as 91 million miles. But we're closest to the sun in January, normally the Northern Hemisphere's coldest month. The reasoning that "the closer we are to the source of heat, the warmer we are" just doesn't wash when it comes to seasons.
What does matter is how directly the sunlight hits our part of the Earth and how long the sun is in the sky. When the Northern Hemisphere is tilted toward the sun, the sun's rays are hitting us more directly and the sun is in the sky for a much longer period of time. This double whammy is called summer.
Right now Earth is in a cooling off period. The days have been getting shorter and will continue to do so until the date of the Winter Solstice in December. The sunlight will become less direct, the sun will be in the sky for a shorter period of time, and all the heat the Earth has absorbed and stored will gradually dissipate as we move from fall into winter.
Cause of tilt: But just how did Earth get tilted in the first place? The answer may be as close as the moon.
The solar system has a tumultuous childhood. Dust particles and gas left over from the formation of the sun collided and grew bigger, sweeping up more and more material until the collisions between larger and larger "worldlets" became cataclysmic events.
Late in its childhood the Earth suffered one of these massive collisions. A body thought to be about the size of Mars collided with our still molten, malleable planet. The impact may have knocked Earth into its current tilt; it probably also formed our moon by knocking loose a large portion of our surface.
If that collision had never taken place, we wouldn't have our glorious moon. We also wouldn't have seasons.
XFor more about the reason for the seasons, check Phil Plait's "Bad Astronomy" page at www.badastronomy.com/bad/misc/seasons.html. For kids, there's an easy explanation at the NASA Kids page:http://kids.msfc.nasa.gov/news/2000/news-autumnalequinox.asp. If you're interested in learning more about Earth's motion and astronomy in general, there's a great non-technical textbook on-line called "From Stargazers to Starships" at www.istp.gsfc.nasa.gov/stargaze/Sintro.htm.For more about the solar system, one of the best one-stop spots is Bill Arnett's The Nine Planets at seds.lpl.arizona.edu/nineplanets/nineplanets/.