Forget everything you hear and read about today's astronomical significance, as today is not the first day of fall. Today marks the autumnal equinox. Fall began three weeks ago, and any other opinion is wrong.

The autumnal equinox occurs this evening a little after 10:30 PM Eastern Time, which is (you've got to take their word for it) the exact moment that the sun's rays are shining directly on the equator. After today and until the end of March, direct sunlight will shine in the southern hemisphere, warming up our friends below the equator through their spring and summer months. We, meanwhile, will continue our plunge into chilly bitterness while sipping on society's latest obsession with pumpkin spice.

This, my friends, marks the autumnal equinox. Today is not the first day of fall, nor is today equal parts day and night (no, that doesn't happen for most of the United States until Thursday or Friday). The most interesting factoid about the autumnal equinox is that after today, the sun will rise in the southeast and set in the southwest until the vernal equinox in March.

There are two seasons: meteorological and astronomical. Equinoxes and solstices mark the beginning (and end) of astronomical seasons. Meteorological seasons, on the other hand, begin on the first day of every third month, splitting the year into four seasons each three months long. Meteorological fall began on September 1, ending on November 30; meteorological winter begins on December 1 and stretches through February 20somethingth, and so on for spring and summer.

Most kids in the north begin school in September, and you never see "back to school" clip art with green trees and comical thermometers blowing their tops in front of a sunglassioed sun. It's always a kid wearing a coat running to a bus through leaves of different colors. Halloween candy and fall decorations (pumpkins, scarecrows, hay bales, decorative apple baskets) all suddenly appear in Walmart the day after Labor Day. All that pumpkin spice stuff began—surprise!—at the beginning of the month. The beginning of September is when we hit our figurative and literal sociological fall.

Not only do our societal quirks dictate that fall begins on the first of September, but meteorology and climatology do as well. Let's send regular readers into a meltdown and use New York City as an example. Below is a graph of average annual temperatures in Central Park for 1981-2010.

Here is what that graph looks like with astronomical seasons overlaid on top:

And here's what it looks like with meteorological summer overlaid:

It doesn't seem like 20 or 21 days makes a big difference in when the seasons start, but for average temperature trends in the mid-latitudes, it makes a huge difference. Meteorological seasons—starting at the first of the month instead of in its fourth week—better relate to the average temperature curve seen in most places in the United States. Spring starts as the temperature slowly but surely ticks up from chilly to comfortable. Summer starts on June 1 as we reach the hot hump in the year. Fall begins on September 1 just as we start our dip into cooler air, and winter on December 1 as we bottom out on the temperature curve.

Astronomical seasons are a mess when you try to use them to judge temperatures. If you follow to the earth's orbit for your seasons instead of the arbitrary man-made calendar, your summer begins about a week before your reach the hottest part of the year. That makes no sense.

Today is not the first day of fall, and neither is tomorrow (as most calendars suggest). Fall began three weeks ago. Today is just an astronomical road sign on our cosmic trip.

[Images: AP, xmACIS2]