Curious why this happens? WIRED spoke with Tansu Daylan, a former Transiting Exoplanet Survey Satellite (TESS) postdoctoral associate at MIT, to better understand the winter solstice and our planet’s relationship with the sun. To picture what is happening in space during a winter solstice, start by thinking of a giant glass ball surrounding the Earth, and let’s ignore the planet’s rotation (which complicates everything). Daylan says, “If you look at the three-dimensional sphere that’s around us, known as the celestial sphere, the sun as well as all other solar system objects move through a plane in this celestial sphere, known as the ecliptic plane.” “While doing so, the sun changes its declination,” he says. Declination and right ascension are the two primary axes of the celestial sphere. “In this reference frame, the sun is at the most southern location when it’s the winter solstice from our perspective in the northern hemisphere.” A winter solstice in the northern hemisphere happens when the north pole is tilted farthest away from the sun. The Arctic Circle is cloaked in darkness and experiences its longest night of the year. In the southern hemisphere, a summer solstice simultaneously happens with the south pole angled toward the sun and the Antarctic Circle experiencing midnight sun. NASA’s online Basics of Space Flight tutorial includes a section on the celestial sphere with illustrations for anyone who would enjoy exploring the idea further. The low position of the sun during a winter solstice will cause your midday shadow to be exceptionally long. Even though it is important to humans, a solstice does not have much meaning in regard to the larger cosmos. “The solstices are defined with respect to the Earth-sun system, not necessarily the whole solar system. We attach a lot of meaning to it because the sun is so sacred for us, and its location on the celestial sphere, as a function of time throughout the year, is very important,” Daylan says. “It sets the climate. Throughout the year, it tells us when the crops will mature. So that’s very important, especially to historical civilizations.” From the yuzu baths in Japan to the Inti Raymi celebrations in Peru, cultures around the world have a long history of celebrating the winter solstice. Contemporary Druids and archeologists in England continue to fixate on the Stonehenge monument during this time of year. The two groups’ interests align on occasion, but they often disagree over issues like the display of human remains found at Stonehenge. Although the holiday is celebrated earlier in December, St. Lucia Day in Scandinavia traces its origins back to the winter solstice. Buttery saffron buns twisted into an S-shape and accented with raisins are consumed as part of St. Lucia Day activities. In Iran and surrounding countries, families come together during the winter solstice to celebrate Shab-e Yalda. Reflecting on the Yalda nights from her childhood, Iranian-born food writer Shadi HasanzadeNemati says that over the solstice, “everybody stays up late, and we spend the night reading poetry, having nuts, and eating fruits such as watermelon.” She told stories about her father searching for the ripest, reddest watermelon to eat, and laughed about not getting her expectations too high due to the fruit being out of season. “The red color can represent the start of a new day and dawn,” she says. Pomegranates are another popular Yalda night treat. “My dad would cut them open and separate the seeds. We always had this really big bowl of pomegranates.” HasanzadeNemati suggests making her khoresht fesenjan recipe (pomegranate and walnut stew) as a warm meal to enjoy on a chilly December solstice. The stew might appear intimidating at first, with pomegranate molasses on the ingredient list, but the ingredient is possible to make at home if it is not available at a nearby store. In addition to sharing delicious food with those you love, the winter solstice is an opportunity for introspection. Whether you watch it on December 21 from Stonehenge or just an apartment balcony, breathe in deep and take a moment to reflect on the past year. Acknowledge the hardships encountered, but remain resolute with your eyes on the future. More sunshine is headed our way.
