In 2022, winter the solstice of the northern hemisphere occurs on December 21. Although not as interesting as the 2020 conjunction of Jupiter and Saturn, the longest night of the year is a time of important transition. The winter of the stars is beginning, and the days are slowly getting longer again.
Want to know why this happens? WIRED spoke with Tansu Daylan, a former Transiting Exoplanet Survey Satellite (TES) fellow at MIT, to better understand the winter solstice and our planet’s relationship with the sun.
To visualize what happens in space during the winter solstice, first imagine a giant glass ball around the Earth, and ignore the rotation of the planet (which includes everything). Daylan says, “If you look at the three-dimensional sphere around us, known as the celestial sphere, the sun and all the other objects in the solar system travel in a plane on this sphere, known as the ecliptic plane.”
He says: “When it does that, the sun changes its setting.” Right ascension and descent are the two main axes of the celestial sphere. “In this frame of reference, the sun is at its southernmost position when it is winter from our point of view in the northern hemisphere.”
The winter solstice in the northern hemisphere occurs when the north pole is tilted farthest from the sun. The Arctic Circle is shrouded in darkness and is enjoying its longest night of the year. In the southern hemisphere, the summer solstice is the same time the south pole faces the sun and the Antarctic Circle receives the sun at midnight.
NASA’s online course Basics of Space Flight includes a section on the celestial sphere with illustrations for anyone who would enjoy exploring this concept further.
The lower position of the sun during the winter solstice will cause your daytime shadow to be much longer. Although important to humans, the solstice does not have much meaning in terms of the larger cosmos.
“The solstices are defined in relation to the Earth and sun system, not the entire solar system. We add a great meaning to it because the sun is very sacred to us, and its place in the celestial sphere, as a function of time throughout the year, is very important,” said Daylan. “It sets the mood. Throughout the year, it tells us when the plants will ripen. So that is very important, especially in historical civilizations.”
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