The first day of summer officially kicked off today at 7:28 a.m. ET, the beginning of the summer solstice and the longest day of the year in the Northern Hemisphere.
The summer solstice is a result of Earth's north-south axis being tilted 23.5 degrees relative to the sun, experts say. This tilt causes different amounts of sunlight to reach different regions of the planet over the course of the year. 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.
Photograph by Konstantin Zavrazhin, Getty Images
Solstice Celebration Circle
Followers of Slavic neo-pagan religious movements gather in a Russian forest for a summer solstice 2010 celebration on Saturday.
The summer solstice is recognized and often celebrated in many cultures around the world—in both the past and present, experts say. For instance, Stonehenge in the United Kingdom has been associated with the winter and summer solstices for about 5,000 years.
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. But that doesn't mean the first day of summer is also the hottest day of the year.
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.
Summer solstice 2010 revelers sit in a "friendship nest" at the Body and Soul festival, held at Ballinlough Castle in County Meath, England, on Sunday.
The summer solstice has long been marked by cultures worldwide. 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.
Photograph by Niall Carson, PA Wire/AP
Summer's First Sunrise
The sun rises behind a rocky crest filled with astronomical markers at the Kokino observatory in Macedonia early on Sunday, the day before the summer solstice.
The 4,000-year-old astronomical observatory includes special stone markers used to track the movement of the sun and moon on the eastern horizon.
Photograph by Robert Atanasovski, AFP/Getty Images
Indian New Year
Bolivian indigenous people celebrate sunrise during a winter solstice ceremony in Tiwanaku, about 44 miles (70 kilometers) from the capital of La Paz, on Monday.
In the Southern Hemisphere, the June solstice is the first day of winter, since that part of Earth is tilted farthest from the sun.
The winter solstice coincides with the start of the New Year for South America's Aymara Indians.
Photograph by David Mercado, Reuters
Cloudy Summer Sunrise
Thousands of druids and revelers witnessed a cloud-obscured sunrise Monday at Stonehenge on summer solstice 2010.
Many people make an annual pilgrimage to the ancient site, on England's Salisbury Plain, to celebrate the first day of summer.