Friday, December 21, at 6:11 a.m., is the Winter Solstice for 2012. This is the moment when the Sun reaches the Tropic of Capricorn, its lowest point in the Southern Hemisphere. Though the weather feels wintry in most regions for several weeks before this day, the Solstice marks the official start of the winter season by contemporary reckoning.
The term “solstice” comes from the Latin words “sol” (sun) and “sistere” (to stand still) because, during the solstice, the angle between the Sun’s rays and the plane of the Earth’s equator (called declination) appears to stand still.
So what does that mean? Essentially, our hours of daylight — the period of time each day between sunrise and sunset — have been growing slightly shorter each day since the Summer Solstice in June, which is the longest day of the year (at least in terms of light). Starting after December 21, the days will begin to grow longer and will continue to do so until we reach the Summer Solstice on June 21, 2013, at 1:04 a.m., and begin the whole cycle anew!
While we celebrate the Winter Solstice, our friends in the Southern Hemisphere will be simultaneously marking the Summer Solstice. That’s because, while our half of the globe is inclined away from the Sun, their half is inclined toward it. Starting this week, their days will begin to grow shorter.