In 2021, a The winter solstice occurs in the Northern Hemisphere on December 21. Although not as dramatic as the conjunction of Jupiter and Saturn in 2020, the longest night of the year is an important turning point. Astrological winter begins, and the days are gradually increasing again.
Curious why this happens? Wired spoke with Tansu Dylan, a postdoctoral fellow in the Transiting Exoplanet Survey Satellite (TESS) at MIT, to better understand the winter solstice and our planet’s relationship to the sun.
To visualize what happens in space during the winter solstice, start by thinking of a giant glass ball surrounding the Earth, and let’s ignore the planet’s rotation (which complicates everything). Dylan says, “If you look at the three-dimensional sphere around us, known as the celestial sphere, the Sun and all the other solar system objects move through a plane in this celestial sphere, known as the ecliptic plane.”
“While doing this, the sun changes its declination,” he says. Declination and right ascension are the two main axes of the celestial sphere. “In this frame of reference, the sun is at its southernmost position when the winter solstice is from our perspective in the northern hemisphere.”
The winter solstice occurs in the Northern Hemisphere when the North Pole tilts away from the Sun. Darkness envelopes the Arctic Circle and you experience the longest night of the year. In the Southern Hemisphere, the summer solstice occurs simultaneously with the South Pole at an angle toward the Sun and the Antarctic Circle exposed to the midnight sun.
The NASA Basics of Space Flight tutorial includes a section on the celestial sphere with illustrations for anyone who might enjoy exploring the idea further.
The sun’s low position during the winter solstice will make for an exceptionally long midday shadow. Although important to humans, the solstice doesn’t make much sense in relation to the larger universe.
“solstices are defined in relation to the Earth-Sun system, not necessarily the entire solar system. We attach a lot of meaning to them because the Sun is very sacred to us, and its position on the celestial sphere, as a function of time throughout the year, is very important,” says Dylan. “It adjusts the climate. Throughout the year, it tells us when the crops are ripe. So this is very important, especially for historical civilizations.”
From yuzu baths in Japan to Inti Raymi celebrations in Peru, cultures around the world have a long history of celebrating the winter solstice. Druids and contemporary archaeologists in England continue to focus on Stonehenge during this time of year. The interests of the two groups sometimes coincide, but they often differ on issues such as the presentation of the human remains found at Stonehenge.
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Many winter solstice traditions involve the use of food to bring families and communities together. A version of mince pie may have been eaten during the Stonehenge holidays. China’s Dongzhi festival is famous for its tangyuan, which is rice dumplings often stuffed with black sesame paste.
Although the holiday is celebrated as early as December, Saint Lucia’s Day in Scandinavia traces its origins back to the winter solstice. S-shaped saffron rolls decorated with raisins are consumed as part of Saint Lucia’s Day activities.
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