The Northern Hemisphere's autumnal equinox—the first day of fall—occurs this Saturday, September 22.
The march of the seasons—winter, spring, summer, and fall—stems from the "clearly definable" position of the sun on the summer and winter solstices, according to Judith Young, a professor of astronomy at the University of Massachusetts in Amherst.
"The solstices are very accurately measured as the northernmost point that the sun rises along the horizon in June and the southernmost point along the horizon in December," she said. "It doesn't matter where you are on Earth—that's true."
This regularity allowed for the construction of Stonehenge in England some 5,000 years ago, where sunrise on the summer solstice is still celebrated with fervor.
In modern times, the solstice points became the astronomical definitions of when the summer and winter seasons begin. In the Northern Hemisphere, June features the summer solstice, while in the Southern Hemisphere, June marks the first day of winter.
Equinoxes and Seasons
Since the equinoxes fall roughly halfway between the solstices, they got pegged as the starts of the other two seasons, fall and spring, Young said.
However, the autumnal equinox and vernal—or spring—equinox aren't exactly midway between the solstices "because the Earth's orbit is not a true circle. We have a slightly elliptical orbit," Young explained.
This elongated orbit means that Earth goes faster around the sun in January, when it's closest to the star, than it does when it's farthest away from the sun in July.
"We arrive at the September equinox a day late, because we were going a little bit slower in July, and we arrive at the March equinox a day earlier," Young said.
But our late arrival doesn't make the first day of fall any less special. (See "Autumnal Equinox Pictures: Rituals of Fire and Light" .)
For instance, the spring and fall equinoxes are the only two times during the year when the sun rises due east and sets due west, according to Alan MacRobert, senior editor of Sky & Telescope magazine.
The autumnal equinox and vernal equinox are also the only days of the year when a person standing on the Equator can see the sun passing directly overhead.
On the Northern Hemisphere's autumnal equinox, a person at the North Pole would see the sun skimming across the horizon, signaling the start of six months of darkness.
On the same day, a person at the South Pole would also see the sun skim the horizon, beginning six months of uninterrupted daylight.
(See pictures of the sun's path across the sky—an entire year in a single frame.)
The Lag of the Seasons
The defined start of the seasons based on the sun's positions may seem counterintuitive. After all, in the summer, daylight begins to grow shorter just as the season officially begins.
Shouldn't the June solstice instead be called midsummer, as was celebrated in Shakespearean times?
From a climatological perspective, the answer is no, according to Young, who explained that "there's something called the lag of the seasons where [for example] the temperatures continue to warm up after you've had the northernmost sunrise in the Northern Hemisphere" on the summer solstice.
This lag means that, in the Northern Hemisphere, the warmest days of summer don't actually arrive until late July and early August, and the coldest days of the winter are in January and February.
"Because of that lag, it actually made climatological sense to define the seasons as starting when we do," Young said.
Autumnal Equinox Illusions
But don't be fooled by the notion that on the autumnal equinox the length of day is exactly equal to the length of night.
The true days of day-night equality always fall after the autumnal equinox and before the vernal equinox, according to Geoff Chester, a public affairs specialist with the U.S. Naval Observatory in Washington, D.C.
The difference is a matter of geometry, atmosphere, and language.
Day and night would each be exactly 12 hours long on a spring or fall equinox only if the sun was a single point of light and Earth had no atmosphere.
But the sun, as seen from Earth, is nearly as large as a little fingertip held at arm's length—a size known to astronomers as half a degree wide.
Sunrise is defined as the moment the top edge of the sun appears to peek over the horizon. Sunset is when the very last bit of the sun appears to dip below the horizon.
The vernal and autumnal equinoxes, meanwhile, occur when the center of the sun's disk crosses what's known as the celestial equator, an imaginary line that projects outward from Earth's Equator, Chester noted.
What's more, Earth's atmosphere bends sunlight when it's close to the horizon, making the sun appear to rise a few minutes earlier than it actually does.
"Those factors all combine to make the day of the equinox not the day when we have 12 hours [each] of light and darkness," Chester said.
Most people will never see the full 12 hours of sunup and sundown on the autumnal equinox, the University of Massachusetts' Young added.
That's because most people have hills or trees blocking their views of a flat horizon. Thus, they see the sun rise later and set earlier than it does for a horizon without obstruction, she said.
What's more, for people who don't live on the Equator, the sun still rises and sets at an angle to the horizon, noted Young, who built a Stonehenge-like solar calendar and observatory on the University of Massachusetts campus.
Even though the sun rises due east and sets due west on the autumnal equinox, "you'll only see an east sun rising and west sun setting with an obstruction-free horizon," she said.
Another equinox oddity: A rule of the calendar keeps spring almost always arriving on March 20 or 21—but sometimes on the 19th—Sky & Telescope's MacRobert said.
In 1582 Pope Gregory XIII established the Gregorian calendar, which most of the world now observes, to account for an equinox inconvenience.
If the pope hadn't established the new calendar, every 128 years the spring equinox would have come a full calendar day earlier, eventually putting Easter in chilly midwinter.
"It begins with the fact that there is not an exact number of days in a year," MacRobert said.
Before the pope's intervention, the Romans and much of the European world marked time on the Julian calendar.
Instituted by Julius Caesar, the old calendar counted exactly 365.25 days a year, averaged over a four-year cycle. Every four years a leap day helped keep things on track.
It turns out, however, that there are 365.24219 days in an astronomical "tropical" year—defined as the time it takes the sun, as seen from Earth, to make one complete circuit of the sky.
Using the Julian calendar, the seasons were arriving 11 minutes earlier each year. By 1500 the spring equinox had fallen back to March 11.
To fix the problem, the pope decreed that most century years (such as 1700, 1800, and 1900) would not be leap years. But century years divisible by 400, like 2000, would be leap years.
Under the Gregorian calendar, the year is 365.2425 days long, "close enough to the true fraction that the seasons don't drift," MacRobert said.
With an average duration of 365.2425 days, Gregorian years are now only 27 seconds longer than the length of the tropical year—an error which will allow for the gain of one day over a period of about 3,200 years.
Nowadays, according to the U.S. Naval Observatory's Chester, equinoxes migrate through a period that occurs about six hours later from calendar year to calendar year, due to the leap-year cycle.
The system resets every leap year, slipping a little bit backward until a non-leap century year nudges the equinoxes forward in time once again.