What’s the Winter Solstice, Anyway?

Welcome to the official beginning of astronomical winter, which is marked by the winter solstice. WIRED spoke with Tansu Daylan, a former Transiting Exoplanet Survey Satellite 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. 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. “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.