In Ohio, the December solstice comes on Dec. 21 at 6:03 p.m. EST. That's Dec. 21 at 23:03 Universal Time (UTC) or Greenwich Mean Time (GMT). It marks the Northern Hemisphere's shortest day (first day of winter).

The solstice is an astronomical event, caused by Earth's tilt on its axis, and its motion in orbit around the sun.

Because Earth doesn't orbit upright, but is instead tilted on its axis by 23-and-a-half degrees, Earth's Northern and Southern Hemispheres trade places in receiving the sun's light and warmth most directly. The tilt of the Earth – not our distance from the sun – is what causes winter and summer. At the December solstice, the Northern Hemisphere is leaning most away from the sun for the year.

The December solstice occurs when the sun reaches its most southerly declination of -23.5 degrees. In other words, when the North Pole is tilted furthest – 23.5 degrees – away from the sun.

All locations south of the equator have day lengths greater than 12 hours at the December solstice. Meanwhile, all locations north of the equator have day lengths less than 12 hours. So, the December solstice marks the shortest day of the year in the northern hemisphere and longest day in the southern hemisphere.

For us in the north, today is the day the sun stands still (the literal meaning of "solstice"). Today is the day it gets as low as it can at midday-that's why it "stands still"; it's dipped as low as it can go and has stopped its decline. It's the shortest day and longest night of the year. If you go out tomorrow it will be a wee bit higher at midday, and the day will be a tad longer.

The change is slow at first, then speeds up, accelerating the most at the vernal equinox in March. On that day, the days are lengthening as quickly as they can, usually by a couple of minutes or so per day. Then, at the June solstice, the sun is as high in the sky as it can get, days are at their maximum length, and the sun stands still once again. It reverses course, and starts getting lower every day at midday until late December.

In Ohio, the December solstice comes on Dec. 21 at 6:03 p.m. EST. That's Dec. 21 at 23:03 Universal Time (UTC) or Greenwich Mean Time (GMT). It marks the Northern Hemisphere's shortest day (first day of winter) and Southern Hemisphere's longest day (first day of summer). Happy solstice, everyone!