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The first day of summer officially kicks off today at 7:28 a.m. ET, the beginning of the summer solstice and 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 23.5 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 closer to the sun than on any other day of 2010. The opposite holds true for the Southern Hemisphere, for which 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 James Bell, an astronomer at Cornell University in New York State.
It depends on the viewer's latitude. "The sun is only shining down directly overhead [at noon] at the Equator. 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.
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 does change every once in a while. 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 365.25 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 solstice sunset, when viewed from the Sphinx, sets precisely between two of the Pyramids.
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.)
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 watch the summer solstice sun rise over the Heel Stone, which stands just outside Stonehenge's stone circles.
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