Winter is coming. Our ancestors celebrated the winter solstice, the longest night of the year, with festivals marking the cycle of death and rebirth. Since then, astronomers have determined the scientific basis for Earth’s seasons, and even discovered winters on other planets. This year, snuggle up under a cozy blanket with a toasty cup of hot cocoa and learn more about the science and culture behind this annual event.

Original photo by Pipop_Boosarakumwadi/ iStock

The Winter Solstice Occurs at a Specific Moment

Close up of a vintage globe with illustrations and latin text. — Credit: C Salisbury/ Shutterstock

Most people observe the winter solstice as a day on the calendar — usually December 21 or 22 in the Northern Hemisphere — that marks the beginning of the coldest season. Astronomically speaking, however, the winter solstice occurs when the sun appears directly overhead at noon at the Tropic of Capricorn. This invisible line circles the planet at 23°26’22” south latitude and runs through Paraguay, Namibia, Australia, and other countries. The winter solstice corresponds with the exact moment the Northern Hemisphere reaches its maximum tilt away from the sun.

In the Southern Hemisphere, the timing and location are reversed. The winter solstice there occurs around June 20 or 21 at the moment when the sun appears overhead at noon at the Tropic of Cancer, which circles Earth at 23°26’22” north latitude (crossing Mexico, Saudi Arabia, India, and other places). The timing coincides with the Southern Hemisphere’s maximum tilt away from the sun.

The Winter Solstice Marks the Shortest Day and Longest Night of the Year

Polar winter dusk landscape. — Credit: zlikovec/ iStock

When either hemisphere is tilting away from the sun to its greatest possible extent, people who live there experience the least daylight and most night of the year. But just how short the day is depends on your precise location — specifically, how close you are to the equator. More southerly locations in the Northern Hemisphere have longer shortest days; the shortest days in northerly locations last only a few hours. For example, Manila in the Philippines lies near the equator and experiences 11 hours, 15 minutes, and 57 seconds of daylight on the winter solstice. In Reykjavik, Iceland, daylight lasts just four hours, seven minutes, and nine seconds.

Winter Starts Earlier for Meteorologists

Meteorologists in a weather monitoring room. — Credit: Kim Steele/ Photodisc via Getty Images

Astronomers and meteorologists have different criteria for determining the seasons. Technically, the cold season that begins on the winter solstice is astronomical winter, determined by the position of the sun. Astronomical spring, summer, and fall begin on the vernal equinox, summer solstice, and autumn equinox, respectively.

But the varying dates of the equinoxes and solstices make it difficult to compare weather data from one year to the next, so weather forecasters typically use meteorological seasons based on temperature averages and the regular calendar. In the meteorological calendar, each season is assigned a three-month period: Winter is December through February, spring is March through May, summer is June through August, and fall is September through November.

Ancient Monuments May Mark the Winter Solstice

Newgrange, a prehistoric monument built during the Neolithic period. — Credit: MNStudio/ Shutterstock

Human civilizations around the world built monuments that may have noted the change of seasons as they related to planting and harvesting crops. Newgrange, a 5,000-year-old dome-shaped monument in Ireland, features a small opening in its roof through which the sun shines on the winter solstice. Stones at Stonehenge, built around 2500 BCE in southern England, are aligned to the sun’s angles on the summer and winter solstices. Indigenous peoples in North America also built monumental calendars. The Chaco Culture of northwest New Mexico recorded the winter solstice at special observatories built between 800 and 1100 CE. The Mississippian Culture established “Woodhenge,” a calendar of upright logs, around 1000 CE at Cahokia in modern-day Illinois. In southeast Ohio between 100 BCE and 400 CE, the Hopewell Culture built earthworks that aligned with the rising sun at the winter solstice.

Many Cultures Observe the Winter Solstice With a Festival

Saturnalia, the December festival of Saturn in ancient Rome. — Credit: North Wind Picture Archives/ Alamy Stock Photo

Winter solstice celebrations often note the passage of time with feasts and gatherings. People in ancient Rome held Saturnalia, a holiday on which social conventions were turned upside down to honor Saturn — the god of the harvest — and the conclusion of the planting season. The Venerable Bede, an English monk who lived in the eighth century, wrote about the Germanic winter solstice festival called Yule; over time, Europeans blended Yule and Christmas traditions. Celebrants of the Chinese festival Dongzhi observe the winter solstice by making holiday foods, including tangyuan (glutinous rice balls served in a sweet broth), wontons, nuts, and soups. Scientists in Antarctica celebrate Midwinter Day in June with a holiday meal, athletic events, and movie marathons.

Other Planets Have Winter Solstices

Realistic space background with all planets of the solar system. — Credit: Macrovector/ Shutterstock

Each planet in the solar system has seasons determined by the tilt of its axis and the shape of its orbit, just like on Earth. Smaller tilts and more circular orbits correspond with less-noticeable seasons: On Venus and Jupiter, with minimal tilts and roundish orbits, summer and winter are pretty similar. On Mars, the tilt of 24 degrees and oval orbit give it dramatic seasonal shifts. Seasons on the solar system’s outer planets are not well understood, but scientists do know their winters last a lot longer than on Earth — about seven years on Saturn, 20 years on Uranus, and more than 40 years on Neptune.

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