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Observers in Macedonia watch the sun rise at the stone observatory Kokino.
Macedonians greet the dawn on summer solstice 2011 at the ancient observatory of Kokino.

Photograph by Georgi Licovski, European Pressphoto Agency

Ker Than

for National Geographic News

Updated June 21, 2011

The first day of summer—heralded today by a manic bunny and bear in a Google doodle by artist Takashi Murakamiofficially kicks off today at 1:16 p.m. ET, the beginning of the summer solstice and of the longest day of the year, at least in the Northern Hemisphere.

(Pictures: Summer Solstice Marked With Fire, Magic.)

The summer solstice is a result of the Earth's north-south axis being tilted 23.4 degrees relative to the sun. The tilt causes different amounts of sunlight to reach different regions of the planet.

Today the North Pole is tipped more toward the sun than on any other day of 2011. The opposite holds true for the Southern Hemisphere, where today is the winter solstice, the shortest day of the year.

As a result, at high noon on the first day of summer, the sun appears at its highest point in the sky—its most directly overhead position—in the Northern Hemisphere.

That doesn't mean the sun will be exactly overhead at noon for everyone, said Cornel University astronomer James Bell.

It depends on the viewer's latitude—the sun will shine down directly overhead at noon only along the Tropic of Cancer, an imaginary line that circles the Earth at about the latitude of Cuba.

"It's still at a low angle if you're up in Alaska," Bell explained.

No matter where you are in the Northern Hemisphere, the path of the sun across the sky—which rises in the lead-up to the first day of summer, then begins descending over the rest of the summer—seems not to change for the few days before and after the summer solstice. (See pictures of the sun's path across the sky—an entire year in a single frame.)

In reality, the sun's position is still changing, but at a slower rate.

(Related story: "In Scandinavia, Solstice Means Fun in the Midnight Sun.")

Summer Solstice Wobbles Around the Calendar

While the June solstice generally occurs on the same day every year, the date can change from year to year. For example, in 2008, the summer solstice occurred on June 20.

This date shifting is a result of the discrepancy between a human calendar year—which is usually counted as 365 days—and an astronomical year, which is about 365.24 days.

Our leap year system—which adds an extra day to the calendar every four years—ensures our calendars are accurate, but it also causes the solstice date to flop around a bit.

"It's nothing astronomical changing. What's changing is the human side of it," Bell said.

Solstice Is Longest Day of the Year—Not Hottest

On the summer solstice, the Northern Hemisphere receives more sunlight than on any other day of the year, but that doesn't mean the first day of summer is also the hottest day of summer.

Earth's oceans and atmosphere act like heat sinks, absorbing and reradiating the sun's rays over time. So even though the planet is absorbing lots of sunlight on the summer solstice, it takes several weeks to release it. As a result, the hottest days of summer usually occur in July or August.

"If you think about turning up an oven—it takes it a long time to heat up," explained Robert Howell, an astronomer at the University of Wyoming. "And after you turn it off, it takes a while for it to cool down. It's the same with the Earth."

First Day of Summer Sparked Ancient Celebrations

The summer solstice is recognized and often celebrated in many cultures around the world, in both the past and present.

The ancient Egyptians, for example, built the Great Pyramids so that the sun, when viewed from the Sphinx, sets precisely between two of the Pyramids on the summer solstice.

The Inca of South America celebrated the summer solstice with a ceremony called Inti Raymi, which included food offerings and sacrifices of animals and maybe even people. (See a picture of an Inca summer solstice festival.)

Recently, archaeologists discovered the remains of an astronomical observatory in a long-buried Maya city in Guatemala in which the buildings were designed to align with the sun during the solstices. During such times, the city's populace gathered at the observatory to watch as their king appeared to command the heavens.

And perhaps most famously, Stonehenge in the United Kingdom has been associated with the winter and summer solstices for about 5,000 years.

Observers in the center of the standing stones can still watch the summer solstice sun rise over the Heel Stone, which stands just outside Stonehenge's stone circles. (Read about pagans' campaign to enter Stonehenge on the summer solstice and other sacred days.)

This year modern-day Druids will gather at Stonehenge to celebrate the solstice for the first time as members of an officially recognized religion in the U.K., following a controversial vote by the national Charity Commission for England and Wales last fall.

(Related: "First 'Skyscraper' Built to Fight Solstice Shadow?")

Summer Solstice Not What It Used to Be

For many of the ancients, the summer solstice wasn't just an excuse to party or pray—it was essential to their well-being.

Associated with agriculture, the summer solstice was a reminder that a turning point in the growing season had been reached.

"The calendar was very important—much more important than it is now," said Ricky Patterson, an astronomer at the University of Virginia. "People wanted to know what was going to happen, so that they could be ready."

But for many modern cultures—and Americans in particular—the solstices and equinoxes no longer attract the same kind of attention that they once did.

"The only people who really pay attention to what's going on outside on a regular basis still are like the neo-pagans in America and farmers, because it's important for their growing and harvest seasons," said Jarita Holbrook, a cultural astronomer at the University of Arizona in Tucson.

"But we're pretty much an indoor culture at this point … so we have less of a connection to the sky."

Mark Hammergren, an astronomer at the Adler Planetarium in Chicago, said he doesn't feel too bad about the declining significance of the solstices in modern society.

"Ancient cultures and some modern religions pay very, very close attention to certain natural alignments … and there's a lot of mysticism and special supernatural significance attached to them," he said. "The fact that we don't pay attention to that stuff as much anymore, I think, is a rational thing."

The University of Arizona's Holbrook, however, thinks there are certain benefits in keeping the tradition alive.

"Paying attention to the solstices is a way of teaching mathematics, celestial mechanics and astronomy and culture and history," she said. "It is also a pretty good party."

More: See pictures of the last summer solstice >>

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