Here in the northern hemisphere, the days are getting shorter and the nights are getting colder, all leading up to the winter solstice, which marks the shortest day of the year. But astronomically, that's a bit of an oversimplification: Technically, the solstice this month falls on December 21 at 4:28 PM Universal Time, or 9:28 PM Eastern time. That precision is a reflection of the fact that the solstice is actually calculated by a measure other than day length.
Instead, a solstice marks the point during Earth's year when the planet's axis points directly to the sun. The axis is the line through the planet that Earth spins around, connecting the north and south poles. But that line isn't vertical compared to Earth's path along the sun—instead, it's tilted by about 23 degrees, like a more extreme version of the Leaning Tower of Pisa.
That tilt stays steady as Earth orbits the sun, so that during half the year (which happens to fall between March and September), the north pole is pointing toward the sun. During the other half of the year, between September and March, the south pole is pointing toward the sun, with the solstice marking the midway point of this journey.
As a consequence, on December 21, the northern half gets the least solar radiation as it does all year long, and sees the fewest hours of daylight. But that usually doesn't make the solstice the coldest day of the year, or put it in the middle of winter.
Read more: Uranus: Mysterious Crash Gave Planet Its Crazy Tilt and 18 Moons
That's because the weather has a slightly different sense of seasons, called meteorological seasons rather than astronomical seasons. For weather forecasters, winter begins on December 1 and lasts until February 28 or 29. That distinction is partially to make seasons more equal in length and partially to better reflect how temperatures change over the course of the year.
Neither seasons nor solstices are a reflection of the distance between Earth and the sun, although that distance does change a little over the course of the year. But in fact, for 2018, Earth will be closest to the sun on January 3. While it's a noticeable change for astronomers, the difference isn't the sort of thing you could pick up on just by living on Earth's surface.
Of course, in the southern hemisphere where seasons are flipped, the solstice will mark the beginning of the summer and the days will gradually shorten between now and June.
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