Why the Winter Solstice Arrives Today

Winter officially arrives late Wednesday or in the wee hours of Thursday -- depending on the time zone you are in.

The official time corresponds to 12:30 a.m. EST (9:30 p.m. PST, or 5:30 a.m. Universal Time) Thursday (Dec. 22). This is the point when the northern half of our planet will face directly away from the sun.

This means that days, which have up until now been growing shorter in the Northern Hemisphere, will begin to lengthen.

The solstice is tied to the movements of the heavenly bodies, and not to the calendar -- meaning it can fall anywhere from the 21st to the 23rd, depending on where the planets lie. In 2006 and 2007, it was also on the 22nd.

This happens because the Earth rotates on an axis that is tilted by 23.5 degrees, so the planet leans one way or another as it travels around the sun. This doesn't make much difference for folks living around the Earth's equator, but for those of us farther north, or south, this tilt creates seasons.

The winter solstice marks the end of fall and the beginning of winter. During the solstice, the northern half of the Earth is facing away from the sun, hence it will experience its shortest day of the year as the planet rotates.

The effect of this tilt, and of the solstice, depends on your latitude. Everything above the Arctic Circle will remain shrouded in darkness, with no sun that day, and to the north, the North Pole goes without sunlight for months. Farther south in the Northern Hemisphere, the solstice day becomes longer.

The opposite occurs in the Southern Hemisphere. There, the December solstice marks the arrival of summer.

For those of us in the north, the days may begin to grow longer, but the coldest days are still to come. This is because ocean temperatures drive much of the weather on the continents, and they continue to cool in the relative lack of sunlight this time of year.

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