What Is Winter Solstice? Learn About the Shortest Day of the Year

Stonehenge draws hundreds of people every year for the winter solstice. Matt Cardy/Getty Images

Thursday in the northern hemisphere marks the winter solstice, the single day that has the shortest period of daylight of the year. The day is also the summer solstice, or longest day of the year, in the southern hemisphere.

The solstices, and the seasons they mark, are the result of a celestial coincidence. Earth's days are caused by the planet spinning around its axis, the invisible line that would connect its north and south poles. At the same time, the planet is traveling on a yearly stroll around the sun. But the axis isn't quite perpendicular to the path of its orbit, instead it is tilted on its side about 23 degrees, with the axis always pointing the same way into space.

That means that for half the year, the north pole is tilted a bit toward the sun (and experiences summer) and that for the other half of the year, the south pole is tilted a bit toward the sun, giving the northern hemisphere winter. The solstice, the precise time at which the axis and the sun align, marks the halfway point of this period. That time this year is at 4:28 PM Universal Time, or 9:28 PM Eastern time on Thursday.

The solstice also marks the day with the fewest hours of sunlight in the northern hemisphere, giving the solstice the nickname the shortest day of the year. Precisely how short that is varies with how far north or south you are—around the solstice, the sun never truly rises at the north pole.

Read more: When Does Winter Start? The Shortest Day of the Year Is Approaching

While the shortest day can be kind of a bummer, it has a little silver lining: It means that from here, sunlight will stick around longer and longer. And that's why, for millennia, humans have celebrated the solstice—for the return of the sun that it marks.

The most famous solstice celebration is associated with Stonehenge, a giant stone circle constructed in England around 2500 BC at a site that had been symbolically important for about 500 years before then. The monument is carefully positioned so that on the day of the winter solstice, the sun sets through a gap in the stones. It's also aligned to mark the summer solstice in June.