This year, the northern summer solstice falls on June 21 at 11:54 a.m. ET. South of the Equator, this same moment marks the unofficial beginning of winter. Solstices occur at the same time around the world, but their local times vary with time zones.
Solstices occur because Earth’s axis of rotation is tilted about 23.4 degrees relative to Earth’s orbit around the sun. This tilt is what drives our planet’s seasons, as the Northern and Southern Hemispheres get unequal amounts of sunlight over the course of a year. From March to September, the Northern Hemisphere is tilted more toward the sun, driving its spring and summer. From September to March, the Northern Hemisphere is tilted away, so it feels autumn and winter. The Southern Hemisphere’s seasons are reversed.
On two moments each year—what we call solstices—Earth’s axis is tilted most closely toward the sun. The hemisphere tilted most toward our home star sees its longest day, while the hemisphere tilted away from the sun sees its longest night. During the Northern Hemisphere’s summer solstice—which always falls around June 21—the Southern Hemisphere gets its winter solstice. Likewise, during the Northern Hemisphere’s winter solstice—which falls around December 22—the Southern Hemisphere gets its summer solstice.
You can also think about solstices in terms of where on Earth the sun appears. When it’s a summer solstice in the Northern Hemisphere, the sun appears directly over the Tropic of Cancer, the latitude line at 23.5 degrees North. (That’s as far north as you can go and still see the sun directly overhead.) During the Northern Hemisphere’s winter solstice, the sun appears directly over the Tropic of Capricorn, the Tropic of Cancer’s southern mirror image.
Earth is not the only planet with solstices and equinoxes; any planet with a tilted rotational axis would see them, too. In fact, planetary scientists use solstices and equinoxes to define “seasons” for other planets in our solar system.
It’s worth noting, though, that other planets’ seasons don’t climatically equal those on Earth for a few reasons. First, planets vary in their axial tilts: Venus’s axis of rotation is tilted by just three degrees, so there’s much less seasonal difference between the Venusian summer and winter solstices than those on Earth. In addition, planets such as Mars have less circular orbits than Earth’s, which means that their distances from the sun vary more dramatically than ours do, with correspondingly bigger effects on seasonal temperature.
On Earth, our axial tilt plays a much bigger role in governing our annual seasons than our near-circular orbit. Earth makes its closest annual approach of the sun about two weeks after the December solstice, during the Northern Hemisphere’s winter. Earth is farthest from the sun about two weeks after the June solstice, during the Northern Hemisphere’s summer.
Traditionally, solstices have helped mark the changing of the seasons. However, today’s meteorologists officially use temperature records, not solstices or equinoxes, to draw lines between the seasons.
For millennia, cultures around the world have devised ways to celebrate and revere these celestial events. Here are some of the ways the world has honored the solstices.
People watch the midsummer sun as it rises over the megalithic monument of Stonehenge on Salisbury Plain, England. Crowds gathered at the ancient stone circle to celebrate the summer solstice, the longest day of the year in the Northern Hemisphere.
The enigmatic English structure Stonehenge has a special relationship with the solstices. On the summer solstice, the complex’s Heel Stone, which stands outside Stonehenge’s main circle, lines up with the rising sun.
Though it remains unclear precisely how the ancient Egyptians oriented the Great Pyramids at Giza, the giant structures appear to be aligned with the sun. When viewed from the Sphinx, the sun sets between the pyramids of Khufu and Khafre during the summer solstice.
The traditional Scandinavian holiday of Midsummer welcomes the summer solstice with maypole dancing, drinking, and romance.
During the Slavic holiday of Ivan Kupala, long timed to the summer solstice, people wear floral wreaths and dance around bonfires. Some plucky souls jump over the fires as a way of ensuring good luck and health.
On June 24, in time with the Southern Hemisphere’s winter solstice, the Inca Empire celebrated Inti Raymi, a festival that honored the Inca religion’s powerful sun god Inti and marked the Inca new year. The festival is still celebrated throughout the Andes, and since 1944, a reconstruction of Inti Raymi has been staged in Cusco, Peru, less than two miles from its Inca Empire home.
The people of Fairbanks, Alaska, can get up to 22.5 hours of daylight in the summer, so they swing in the summer solstice with a nighttime baseball game. The Midnight Sun Game has been played 114 times now since 1906. Twenty-one of the games have lasted past 1:00am; the 2018 game ended in a walk-off home run.
Iranians celebrate Yalda, a festival timed to the Northern Hemisphere’s winter solstice in December. The festival—a mainstay since Zoroastrianism was Iran’s dominant religion—traditionally honors the birth of Mithra, the ancient Persian goddess of light.
Ancient Romans celebrated the winter solstice with Saturnalia, a seven-day festival that involved giving presents, decorating houses with plants, and lighting candles.
If solstices mark the brightest and darkest days of the year, why don’t temperatures reflect that? In short, it’s because it takes time for Earth’s land and water to heat up and cool down. In the U.S., the year’s coldest temperatures set in after-mid January, roughly a month after the Northern Hemisphere’s winter solstice. Likewise, thermometers hit their high in the U.S. in July and August, weeks after the summer solstice.
You also may have heard that since Earth’s rotation is slowing down, each new solstice sets a new record for daytime length. But that’s not the case. It’s certainly true that Earth’s rotation has slowed over billions of years, as Earth loses angular momentum to our planet’s tides. Growth lines on fossil corals show that more than 400 million years ago, days on Earth lasted less than 22 hours.
But Earth’s gradual slowing down isn’t the only factor at play. Picture a figure skater twirling on their skates; they can speed up or slow down their twirls by how much they tuck in their limbs. In much the same way, changes in the distribution of Earth’s mass—from the winds of El Niño to the melting of Greenland’s ice—can subtly tweak our planet’s rotation rate.
Taking all this into account, it’s thought that the longest day since the 1830s occurred sometime in 1912. It lasted less than four milliseconds longer than the recent average.
Michael Greshko is a writer for National Geographic’s science desk.
Article originally printed by National Geographic Partners, LLC.