When Is the Summer Solstice 2021
When Is the Summer Solstice 2021 and How Is the First Day of Summer Celebrated?
Earth's tilted rotation brings a shift of seasons and the longest day of the year north of the equator
The summer solstice marks the first day of summer in the Northern Hemisphere, bringing the longest day of the year, the shortest night and, in the northernmost Arctic, a midnight sun that shines around the clock. It coincides with the winter solstice in the Southern Hemisphere, which marks the year's shortest day and the beginning of winter there.
"The solstice is an excellent timekeeping moment, for figuring out where you are in the cyclical nature of the planet," said astrophysicist Jackie Faherty at the American Museum of Natural History in New York. Solstices occur every June and December.
The term "solstice" comes from the Latin words "sol," which means sun, and "sistere" which means to stop. The name reflects the fact that the sun seems to pause briefly when it reaches its annual southernmost or northernmost position in the sky before reversing direction.
"It's called 'solstice' because they are the points in the year where the sun appears to stand still in the sky from the perspective of a watcher watching the sunrise or sunset," Dr. Faherty said.
What is the science behind the solstice?
As it orbits the sun, the Earth traces an ellipse, not a circle, and like a child's top spinning off-kilter, the planet's axis of rotation is tilted by about 23.5 degrees. The summer solstice in the Northern Hemisphere occurs at the moment when the planet's tilt brings the North Pole closest to the sun.
In the months leading up to the Northern Hemisphere's summer solstice, the sun's rays gradually shift northward from the equator until the sun appears directly overhead at noon when seen at 23.5 degrees north latitude, a mapmaker's boundary line called the Tropic of Cancer. At this point, daylight lingers longer than 12 hours everywhere north of the equator and less than 12 hours everywhere to the south.
"The sun's path across the sky is higher in the sky and therefore it's up for more hours," said astronomer Larry Wasserman at the Lowell Observatory in Flagstaff, Ariz. "It's warmer than it is with the winter when the sun is as low as it can be south."
After the summer solstice, the days get shorter each day until the winter solstice in December, when the days begin to lengthen.
"It reverses like a little zigzag that it does back and forth across your horizon from solstice to solstice," said Dr. Faherty.
When exactly does the solstice occur?
While its effects differ on opposite sides of the equator, the solstice takes place at the same moment world-wide.
This year the summer solstice in the Northern Hemisphere falls on Monday June 21 at 03:32 Coordinated Universal Time (UTC), the international standard time used by astronomers. The solstice arrives on Sunday June 20 at 8:32 p.m. Pacific Daylight Time in Los Angeles and at 11:32 p.m. Eastern Daylight Time in New York.
The solstice arrives at the same moment in the Southern Hemisphere, but there it is the winter solstice. Daylight in Sydney on the solstice will last about 9 hours and 53 minutes, which is about 4 1/2 hours less than at the Southern Hemisphere's summer solstice in December.
How have people through the ages celebrated the solstice?
For thousands of years, the solstice has been marked as precisely as technology allowed, first by etching rocks, then by erecting immense stone structures.
On the summer solstice in the Northern Hemisphere, the sun rises directly above the Heel Stone of the prehistoric Stonehenge in England. Two thousand miles away in Egypt, the winter solstice's sunbeams light the inner sanctum of the temple of Karnak, arguably the largest religious structure ever built. At the Mayan pyramid of Chichen Itza in Mexico, the summer solstice sunlight casts perfect shadows on the south and west sides so that the immense structure looks to be split in two. During the spring and fall equinoxes, when day and night are of equal length, the unique pattern of sunlight and shadow makes it seem that a serpent is slithering down the pyramid's grand staircase.
The eve of the 2019 summer solstice was marked at Stonehenge, where the celebration has been canceled this year because of Covid-19.
Photo: Hannah McKay/Reuters"They built monuments aligned to them, not just to measure them but also to commemorate them, to acknowledge what seemed to be the power that they represented in driving the seasons and in defining the environment," E.C. Krupp, director of the Griffith Observatory in Los Angeles, said of the builders of the ancient monuments. Dr. Krupp has studied about 2,200 ancient astronomical sites.
Now astronomers calculate the exact minute of the solstice using a complex mathematical model of the solar system that encompasses the precise orbits of the planets and their moons and hundreds of asteroids, their second-by-second positions and how they influence each other. The data is prepared by the National Aeronautics and Space Administration's Jet Propulsion Laboratory in Southern California.
"What we're really talking about is the continuity of human experience with the sky over their heads and what that has meant for people," Dr. Krupp said. "It's grounding and uplifting."
Modern-day Druids and others by the thousands typically gather at Stonehenge to celebrate the rising sun of the solstice. This year English Heritage, which manages the site, canceled the celebration because of Covid-19 restrictions on public gatherings.
