Solstice

The solstice refers to one of two specific times of the year when the sun reaches its farthest point north or south of the celestial equator. During the summer solstice, which occurs around June 21, the Northern Hemisphere receives its most daylight of the year. This date also marks the beginning of summer. The Northern Hemisphere receives its least amount of daylight during the winter solstice, which occurs around December 21, and marks the start of winter. The solstices are reversed for the Southern Hemisphere, with the summer solstice occurring in December and the winter solstice in June. The name solstice comes from the Latin solstitium, which means “sun standing still.”

Human cultures have noted the importance of both solstices for thousands of years. The summer solstice is often associated with fertility and nature, while the winter solstice welcomes the “rebirth” of the sun. The date on which some people celebrate Christmas has its roots in the winter solstice.

Background

Earth travels around the sun at a speed of 67,000 miles per hour (108,826 kilometers per hour), but humans do not feel that motion from the planet’s surface. From our perspective, it is the sun that appears to be moving against the background stars. The path the sun seems to follow across the sky is called the ecliptic. The sun makes one complete journey through the ecliptic over the course of 365.25 days—the same amount of time it takes Earth to orbit the sun. Along the way, the sun passes through parts of thirteen star patterns, or constellations. The constellations on the ecliptic are known as the zodiac.

Although modern astronomers know Earth is not the center of the solar system, they still use a millennia-old concept to refer to the movement of the sun and planets in the sky. Ancient astronomers who believed the universe revolved around Earth, thought of the sky as a giant celestial sphere. They projected the planet’s geographical points onto this fixed celestial sphere. They designated a celestial equator to correspond with the equator on Earth. Similar to its earthly counterpart, the celestial equator divided the sky into northern and southern hemispheres. At 90 degrees north latitude from the celestial equator, they designated the celestial north pole to correspond with Earth’s North Pole. They did the same with the South Pole.

Overview

As Earth moves around the sun, it also rotates around an imaginary line known as its axis. Earth’s axis is not perfectly straight but is tilted at an angle of about 23.4 degrees. Over the course of a year, the tilt of the axis does not change, but the planet’s position in relation to the sun does. This results in Earth being tilted toward or away from the sun at certain times of the year. The two hemispheres receive either more or less daylight, depending on the direction of the tilt of the axis. During the summer, the Northern Hemisphere is tilted toward the sun; during winter, it is tilted away. The opposite is true for the Southern Hemisphere.

On the summer solstice, the sun reaches its highest point above the celestial equator. In the Northern Hemisphere, the summer solstice usually falls on June 20 or 21; rarely, it can fall on June 19 or June 22. (The date shifts due to the calculation of leap years in the Gregorian calendar.) On this day, the Northern Hemisphere experiences its longest period of daylight. The sun, which appears to have been slowly moving north in the sky each day, reaches its highest point in the sky for the year. At 23.4 degrees north latitude on Earth, the sun appears directly overhead at noon on the summer solstice. This latitude is known as the Tropic of Cancer. It was given the name thousands of years ago when the sun was in the constellation of Cancer at the solstice. The sun is no longer in Cancer on the solstice, but the name is still used.

To ancient cultures, the movement of the sun was an important milestone that allowed them to gauge their planting and harvesting times. They noticed the sun’s position on the summer solstice and marked the occasion with celebrations. Festivals in ancient China celebrated the feminine forces of nature. Native Americans staged elaborate ritual dances to pay tribute to the sun. European cultures held midsummer festivals to honor the goddess of fertility. Many Scandinavian countries still celebrate these festivals. In England, the stone monoliths of Stonehenge—a prehistoric monument built thousands of years ago—are aligned with the rising sun on the morning of the summer solstice. Tens of thousands of people flock to the site on that date to celebrate the start of summer.

After the summer solstice, the sun appears to be moving southward in the sky each day. As it continues its southward movement, the amount of daylight slowly lessens and the amount of darkness grows. This continues until the date of the winter solstice. The winter solstice usually occurs on December 21 in the Northern Hemisphere, but can also occur on December 22; on very rare occasions, it can fall on December 20 or 23. The sun reaches its lowest point below the celestial equator at the winter solstice. As a result, the hemisphere tilted away from the sun experiences the longest nighttime of the year. In northern latitudes, the sun reaches its lowest point above the horizon at noon. At 23.4 degrees south latitude on Earth, the sun appears to be directly overhead at noon. This latitude is known as the Tropic of Capricorn.

After the winter solstice, the sun begins its slow climb back up in the sky. Many ancient cultures celebrated with festivals that symbolized the “rebirth” of the sun and the renewal of its journey. In Scandinavia, people lit great bonfires to welcome back the slow return of the sun’s life-giving warmth. This feast, known as Juul, likely inspired the modern Christmas tradition of lighting a yule log. The Romans celebrated Saturnalia, a seven-day period of feasting held around the solstice in honor of the god Saturn. Many historians believe December 25 was chosen as the date of Christmas to replace the pagan holiday of Saturnalia with a Christian celebration.

Bibliography

Byrd, Deborah. “June Solstice in 2024: All You Need to Know” Earth Sky, 2024, earthsky.org/earth/everything-you-need-to-know-june-solstice. Accessed 21 June 2024.

Byrd, Deborah. “2024 December Solstice: All You Need to Know.” Earth Sky, 2024, earthsky.org/?p=2951. Accessed 21 June 2024.

Matthews, John. The Winter Solstice: The Sacred Traditions of Christmas. Quest Books, 2003.

Schneider, Stephen. “What Is the Summer Solstice? An Astronomer Explains.” The Conversation, 19 June 2023, theconversation.com/what-is-the-summer-solstice-an-astronomer-explains-98270. Accessed 21 June 2024.

Treisman, Rachel. “Summer Officially Arrives with the Earliest Solstice in More than 2 Centuries.” NPR, 20 June 2024, www.npr.org/2024/06/20/nx-s1-5013295/summer-solstice-2024-early. Accessed 21 June 2024.

“What Is the Winter Solstice?” Sky & Telescope, 21 Dec. 2017, www.skyandtelescope.com/astronomy-resources/what-is-the-winter-solstice. Accessed 21 June 2024.

“When Is the Summer Solstice?” Royal Museums Greenwich, www.rmg.co.uk/stories/topics/summer-solstice. Accessed 21 June 2024.