Say goodbye to summer: The Northern Hemisphere's autumnal equinox—the first day of fall—occurs Monday, September 22.
With its brilliant colors and crisper days, autumn is a favorite season for many. But what actually happens to make the seasons change? (See National Geographic's pictures of fall.)
The answer is the "clearly definable" position of the sun on the summer and winter solstices, the late Judith Young, a professor of astronomy at the University of Massachusetts in Amherst, told National Geographic News in 2011.
"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. (See pictures of the sun's path across the sky—an entire year in a single frame.)
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. (See "Summer Solstice Pictures: From Stonehenge to Carhenge.")
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.
Arctic Ice Driven by the Seasons
The natural march of the seasons—winter, spring, summer, and fall—are intricately tied to other natural processes, like the melting and freezing of Arctic sea ice.
If you were standing at the North Pole on the autumnal equinox, for instance, you could see the sun skim across the horizon, signaling the start of six months of darkness.
The careful observer would also note that Arctic sea ice has once again begun to form. (Also see "Shrinking Arctic Ice Prompts Drastic Change in National Geographic Atlas.")
"Once you run out of sun, you run out of energy and there is not much the system can do. You are going to start freezing up," said Mark Serreze, the director of the National Snow and Ice Data Center in Boulder, Colorado.
The minimum extent of Arctic sea ice is on a long-term decline, a trend caused in part by rising global greenhouse gases that's being closely tracked by scientists.
Lag of the Seasons
Defining the 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.
She 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. (Also see "Why Do Fall Leaves Change Color?")
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.
For the same reason, though the peak of solar radiation in the Arctic occurs on the Northern Hemisphere's summer solstice, the maximum melt rate always occurs in July, "because it takes a while for the melt to get going," Serreze said.
The melt rate starts to slow in August as the sun gets lower in the sky. The sea ice minimum usually occurs around September 13, which is a few days before the start of fall.
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, Geoff Chester, a public affairs specialist with the U.S. Naval Observatory in Washington, D.C., explained to National Geographic News in 2005.
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 were 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. (See pictures of fall colors.)
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 noted in 2010.
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 when the horizon has no 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.
Up in the Arctic, the season of darkness is under way, but there's still more warmth now than in past decades, noted Serreze.
That's because the loss of sea ice over the years has exposed more of the ocean, which absorbs the sun's heat. The heat gets released back to the atmosphere as fall sets in.
"We see much warmer conditions in the Arctic than we used to see, and that is really in part because the ocean is moving all of this energy back upward into the atmosphere and into space," he explained.
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