A solstice is an event in which a planet’s poles are most extremely inclined toward or away from the star it orbits.

On our planet, solstices are defined by solar declination—the latitude of Earth where the sun is directly overhead at noon. On Earth, solstices are twice-yearly phenomena in which solar declination reaches the Tropic of Cancer in the north and the Tropic of Capricorn in the south. During the June solstice (marked between June 20 and June 22), solar declination is about 23.5°N (the Tropic of Cancer). During the December solstice (marked between December 20 and December 23), solar declination is about 23.5°S (the Tropic of Capricorn).

Solstices and shifting solar declinations are a result of Earth’s 23.5° axial tilt as it orbits the sun. Throughout the year, this means that either the Northern or Southern Hemisphere is tilted toward the sun and receives the maximum intensity of the sun’s rays. (The only times of the year when the intensity of the sun’s rays is not unequal are the appropriately named equinoxes. During an equinox, solar declination is 0°—the Equator—and both the Northern and Southern Hemisphere receive equal sunlight.)

Sometimes, solstices are nicknamed the “summer solstice” and the “winter solstice,” although these have different dates in the Northern and Southern Hemispheres. The summer solstice is the longest day of the year, meaning it experiences the maximum intensity of the sun’s rays and has the most hours of sunlight. The winter solstice is the shortest day of the year and has the fewest hours of daylight.

The June solstice is the summer solstice in the Northern Hemisphere and the winter solstice in the Southern Hemisphere. The December solstice is the winter solstice in the Northern Hemisphere and the summer solstice in the Southern.

The Science of Solstices

Earth’s latitudes experience the solstices in different ways. At the poles, a solstice is the peak of a radical exposure to daylight, while at the Equator, the solstices are barely marked at all.

Equatorial Regions

The Equator, at 0° latitude, receives a maximum intensity of the sun’s rays all year.

As a result, areas near Earth’s Equator experience relatively constant sunlight and little solstice variation.

Midlatitudes

Earth’s solstices are largely marked by the transition of the subsolar point across the tropics.

The subsolar point describes the latitude where the sun’s rays hit Earth exactly perpendicular to Earth’s surface. It is where the sun appears directly overhead at noon. The subsolar point appears at the Equator twice a year (during the equinoxes), and migrates north and south across the tropics during the rest of the year. The solstices mark when the subsolar point reaches its northernmost and southernmost latitudes.

The sun’s vertical rays strike the Tropic of Cancer, 23.5° north of the Equator, during the June solstice. The subsolar point then begins its migration south, and vertical rays strike the Tropic of Capricorn, 23.5° south of the Equator, during the December solstice. The subsolar point will cross every latitude between these extremes twice every year.

Polar Regions

The subsolar point never reaches Arctic and Antarctic regions. At the North Pole and South Pole, the solstices mark the time when the sun is highest or lowest in the sky. In this way, solstices are the extreme examples of “midnight sun” and “polar night.”

“Midnight sun” describes the phenomenon surrounding the summer solstice, when the sun remains visible at midnight in the weeks leading up to and following the event. The “polar night” surrounds the winter solstice, when the sun remains below the horizon during the weeks leading up to and following the event.

Extraterrestrial Solstices

Every planet in our solar system experiences solstices. The timing and extent of solstices are largely determined by the planet’s axial tilt, orbital eccentricity, and distance from the sun.

Venus, the planet closest to Earth, has a very small axial tilt, just 3°. Venus experiences very little seasonal variation, and its solstices are separated by about three months.

Mars, our other close neighbor, has an axial tilt similar to Earth (24°). However, Mars has a significantly greater orbital eccentricity, meaning it orbits the sun in a more elliptical shape than Earth. As a result of Mars’ larger orbital eccentricity and axial tilt, the Red Planet experiences extreme seasonal variations and its solstices are about 11 months apart.

The Culture of the Solstices

Solstices now mark the beginning of winter and summer, but because some ancient cultures only recognized these two seasons (there was no autumn or spring), the solstices occurred in the middle of the season. Solstices are known as midwinter and midsummer for this reason.

Since ancient times, many cultures have marked the solstices with holidays and festivals.

Midwinter

Followers of many ancient traditions honored the winter solstice, which signaled the cold, winter season. Winter weather put ancient cultures at their most vulnerable; both food and shelter were limited. Many festivities emphasize light, recognizing the winter solstice as the shortest, darkest day of the year.

In many cultures, holidays surrounding midwinter are more prayerful than celebratory—they include cultures praying for survival through the dark and cold, as well as celebrating the spirit of cooperation that helps communities survive difficult times. In other places, midwinter celebrations are the last festivals before the long “famine months” of winter.

Monuments to midwinter holidays can be seen at Stonehenge, in Great Britain, and the so-called Intihuatana Stone (the so-called “hitching post of the sun”) at the Incan ruin of Machu Picchu in Peru. At these sites, people gathered to celebrate and pray for their survival through the rest of winter.

In Japan, midwinter (toji) is marked by traditional yuzuyu hot citrus baths. Yuzuyu are practical as well as symbolic. The hot tubs filled with dozens of citrus fruit are intended to focus prayers for the new year, as well as warm the body and soothe skin chapped by winter winds.

The Sha’lak’o midwinter dance is a custom among the Zuni peoples of what is now the Southwestern United States. During the Sha’lak’o, dancers representing the Zuni fire god and rain god, help the communities bid farewell to the old year and seek blessings in the new.

Perhaps the most famous midwinter celebration is the Saturnalia of Ancient Rome. Saturnalia was celebrated the weeks leading up to the actual solstice. Saturnalia was a wild carnival, as well as a time to mark the passing of the seasons. During Saturnalia festivities, Romans enjoyed banquets, gambling, jokes, gifts, and a tradition of usurping strict social structures. At Saturnalia feasts, masters may have served their slaves, and a “King of Saturnalia” could be appointed to manage merrymaking—decreeing that guests must jump in a river or wear outrageous costumes, for instance.

Early Christians adopted the timing of Saturnalia for two of their most important seasons, Advent and Christmas. Pagans and neopagans, followers of early European religious traditions, still celebrate the winter solstice as a holiday called Yule.

Midsummer

Midsummer heralded the height of agricultural fertility and the slow onset of the harvest season. The growing and harvesting of crops, management of domestic animals, and the ability to hunt wild game were all crucial to the survival of ancient cultures. Midsummer festivals are often celebrations of nature’s bounty.

Ancient European tribes celebrated midsummer with feasts and bonfires, intended to drive away evil spirits. In Alpine and Germanic regions, summer solstice bonfires are so ingrained in the cultures they have their own name: Sonnwendfeuer. Many of these traditions still survive in Europe and countries that have large populations with European (especially Scandinavian) heritage, such as Canada and the United States.

In Finland, midsummer (Juhannus) celebrations include bonfires, saunas, and barbeques. Due to Finland’s proximity to the Arctic, the summer solstice itself can have very little darkness. This makes midsummer in Scandinavia an ideal time for weeklong outdoor music festivals and family vacations.

In Ancient Egypt, the summer solstice signaled the beginning of the new year. Sirius, the brightest star in the night sky, appeared soon after the summer solstice. Egyptian astronomers associated the annual appearance of Sirius with the seasonal flooding of the Nile River, which the civilization depended on for agriculture.

Due to its association with fertility and abundance, midsummer is often associated with romance and marriage. One of the most famous expressions of this romantic sentiment is the comic play “A Midsummer Night’s Dream,” by the English writer William Shakespeare. In the play, fairies enchant two couples in a magical forest—and haphazardly get enchanted themselves. The play’s many role-reversals and changes in appearance are thematically tied to the solstice.