Everything you need to know about the summer solstice
From Stonehenge to the Egyptian pyramids, many historic sites serve as backdrops for celebrating the start of summer in the Northern Hemisphere.
The first day of astronomical summer for the Northern Hemisphere and astronomical winter for the Southern Hemisphere began on June 21, at 5:31 a.m. EDT. Here’s how the changing seasons work.
On June 21, sky gazers gathered at Stonehenge in England to watch the sun rise and a similar ritual will play out in Egypt when the sun sets between the Pyramids of Giza, marking the longest day of the year in the Northern Hemisphere -- the day that the summer solstice occurs.
During the June solstice, the Northern Hemisphere is tilted 23.5 degrees from vertical toward the sun. Direct sunlight reaches its northernmost extent of the year, at Tropic of Cancer, at this time.
From this point, the days begin to shorten as the tilt of the Earth adjusts. When the sun's most direct rays hit the equator on the autumn equinox, the hours of sunlight will equate to the hours of nightfall.
The days continue to shorten until the winter solstice in December, which marks the shortest day of the year for the Northern Hemisphere when it's tilted 23.5 degrees from vertical away from the sun. The sun's most direct rays will fall on the Tropic of Capricorn. Once this passes, the days grow longer in the hemisphere, reaching the spring equinox where once again the length of the daytime will equal the length of the night. This time, however, the days will lengthen until coming to the summer solstice once more.
The opposite occurs for the Southern Hemisphere, reaching the winter solstice in June and the summer solstice in December.
The summer solstice doesn't correlate with the hottest day of the year either, even if the sun is in the sky for a longer period of time.
"Average temperatures typically reach their peak in most temperate locations in July, a few weeks after the solstice as there is a lag-effect between when the sun is at its strongest and when the land surface reaches peak heating," AccuWeather Senior Meteorologist Randy Adkins said.
He compares the growing warmth to cooking a steak on the grill, the meat reaching its peak temperature after it's removed from the rack and "coasting" for a brief period of time.
"The ocean surface takes longer to heat up and cool down, so it coasts longer, reaching its peak temperature in August or even September," Adkins said. "This helps explain why the peak of hurricane season occurs in September – that is when the ocean is at, or close to, its warmest."
Although the summer solstice is not the calendar start of the summer, it is considered the astronomical start of summer.
Due to the tilt of the Earth on the June solstice, there are places in the Northern Hemisphere where the sun either doesn't set or dips briefly below the horizon late into the night. This phenomenon is called the "midnight sun." At the poles, there can be continuous sunlight for up to six months.
The town of Longyearbyen in Svalbard, Norway, is considered one of the northernmost significantly populated cities by the world atlas. Between the end of April to the end of August, the sun does not set for this town, according to a travel website for Norway.
A "polar night" is the opposite of a midnight sun, occurring in the winter where there are days, weeks or even months without the sun rising. While the North Pole experiences a midnight sun, the South Pole will experience a polar night and vice versa.
Nordic countries such as Sweden, Finland, Norway and Denmark hold a Midsummer festival on the June solstice, a halfway point in the midst of the midnight sun before the days begin growing shorter and wane into a polar night.
Other cultures around the world, mostly in ancient times, tracked the solstices through architectural feats. Several monuments line up with the sun on the June solstice.
In this June 21, 2015, file photo, the sun rises as thousands of revelers gathered at the ancient stone circle Stonehenge to celebrate the summer solstice, the longest day of the year, near Salisbury, England. (AP Photo/Tim Ireland, File)
Estimated to have been built around 3,100 B.C., England's Stonehenge aligns with the rising sun of the June solstice. Thousands of tourists flock to the site each year before dawn to catch the alignment of the sun with the Heel Stone of Stonehenge. The Bradshaw Foundation says it took from 3,100 to 2,800 B.C. and three different groups of people to construct the monument.
According to the Old Farmer's Almanac, the stone at the top of Machu Picchu called the Intihuatana, or "the place when the sun gets tied," has a corner that points at each cardinal direction and serves to track the dates of the equinoxes and solstices. The Machu Picchu website says the stone was used as a way for the Incans to indicate when it was time to celebrate the June solstice, which was one of the most important celebrations and rituals of the Incan Empire. Although this does occur during the June solstice, this is considered the winter solstice for the Southern Hemisphere.
In Egypt, an American archaeologist and director of the Giza Plateau Mapping Project, Mark Lehner, made a peculiar observation about the Giza Pyramids. When someone stood directly in front of the Sphinx Temple colonnade during the summer solstice, the individual could see the sun set between the Khufu an Khafre Pyramids. This view of the sun sinking between the pyramids constructs the image of the akhet hieroglyph, which means "horizon," according to the founder of the Glen Dash Foundation for Archaeological Research Glen Dash.
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