Today at 4:48 UTC (Coordinated Universal Time) we marked the winter solstice. (Those of you in Eastern Standard Time experienced it at 11:48 p.m. on December 21.) It's the longest night and shortest day of the year in the Northern Hemisphere. So what is a solstice, and why is it affecting your day?

The Earth doesn't spin vertically like a top as it orbits the Sun. Instead, imagine a line drawn directly through the Earth at its poles. Tilt the Earth along that line at 23.5 degrees. That is the Earth's axial tilt. The direction of the Earth's axial tilt doesn't change as it orbits the Sun (for our timescale, at least), which means the Sun is directly overhead different parts of the Earth at different times of the year. For half the year, the Southern Hemisphere is "closer" to the Sun and its radiation; for the other half, the Northern Hemisphere is closer. You might recognize this phenomenon as the reason we have seasons.

The axial tilt also affects the length of days. Consider a map of the Earth, with horizontal lines dividing it latitudinally, and vertical lines longitudinally. Among the best-known latitudes are the equator, at 0 degrees; the Northern Hemisphere's Tropic of Cancer, at 23.5 degrees; and the Southern Hemisphere's Tropic of Capricorn, at -23.5 degrees. Recognize that number? Now we're getting somewhere.

The winter solstice in the Northern Hemisphere occurs when the Tropic of Capricorn experiences an overhead Sun, which makes that latitude's day 13 hours, 27 minutes long. Because less of the Northern Hemisphere is in direct sunlight, the higher up the Earth you get from the Tropic of Capricorn, the shorter the day. The day is 12 hours long at the equator; just over 10.5 hours at the Tropic of Cancer; and 0 hours at the Arctic Circle, which is in total darkness.

If the lack of sunlight on the shortest day of the year has you weeping into your whiskey, take heart: Beginning tomorrow, the days slowly start to lengthen again. It may be the first day of winter, but spring is on the way. On March 20, 2016, the Sun will be directly "over" the equator. This is the vernal equinox. The Sun is again over the equator in September—the autumnal equinox. And exactly halfway between them is the day that sun worshippers probably consider the best one of the year: the summer solstice, when the Sun is directly over the Tropic of Cancer.