December Solstice

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The Sun is standing still now.

Oh, it's still moving through space, and still rising in the east and setting in the west. But the points along the horizon at which it rises and sets aren't changing. The Sun will rise at the same spot tomorrow as it does today, and the setting points will be the same, too. Thousands of years ago, skywatchers came up with a word to describe this phenomenon: solstice -- a word that means "the Sun stands still."

There are two solstices a year, in December and June. We use the solstices to mark the changing of the seasons. In the northern hemisphere, the December solstice marks the beginning of winter.

Over the course of a year, the Sun appears to move north and south through our sky, so its rising and setting points move north and south along the horizon. The Sun doesn't actually move, though. Instead, the change occurs because Earth is tilted on its axis. At the December solstice, the north pole tilts away from the Sun, so the Sun appears farthest south for the year. But at the June solstice, the north pole tilts toward the Sun, so the Sun stands highest in the north for the year.

The exact time of this year's December solstice is 6:04 a.m. Central Time tomorrow.

Earth's back-and-forth nodding has one other important effect. From here in the north, the Sun is in view for less time at the December solstice than at any other time of the year. That makes the solstice the shortest day of the year.

Script by Damond Benningfield, Copyright 2008

For more skywatching tips, astronomy news, and much more, read StarDate magazine.