Summer has arrived

Source: National Geographic

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.

Yesterday, June 21st, the North Pole was tipped more toward the sun than on any other day of 2011. The opposite holds true for the Southern Hemisphere, where  the winter solstice, is 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.

The above multiple-exposure photo shows the figure 8 path of the sun over the course of the entire year, this is known as an analemma. Photographs are made by taking a picture of the sun from the same place at the same time of day once or twice a week, generating 30 to 50 frames.This one in ancient Delphi, was made in Greece in 2001.

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.

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.

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.

Summer solstice celebrations at Stonehenge.

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.