June 21 (UPI) -- Thursday marked the official start of summer, and sunrise began the longest day of the year in the Northern Hemisphere.
Nearly 10,000 early birds gathered at Stonehenge in Britain at dawn to mark the occasion.
It is the beginning of the summer solstice, the day of the year the North Pole is most inclined toward the sun. As a result, the sun can be seen longer than any other day. The opposite occurs in December for the winter solstice.
In Britain, the sun first appeared at 4:52 a.m.
At Stonehenge, the sun rose behind the Heel Stone entrance to the stone circle and rays of sunlight were channeled into the monument's center. Celebrating the solstice at Stonehenge is a tradition experts say goes back thousands of years.
The monument was built in several stages, with the first believed to be built about 5,000 years ago. The stone circle was erected in the late Neolithic period, about 2500 B.C.
Stonehenge is usually closed to the public, but it opens for the summer solstice. This year, officials said they increased security for the event. No disruptive incidents occurred.