The winter solstice marks the shortest day and longest night of the year.  In the Northern Hemisphere, it occurs when the sun is directly over the Tropic of Capricorn, which is located at 23.5° south of the equator and runs through Australia, Chile, southern Brazil, and northern South Africa. This year, the Northern Hemisphere winter solstice will occur at 4:12 am MST on December 21, 2012. For a complete listing of the dates of the winter and summer solstice's and spring and fall equinox's through 2020, check out this site from the U.S. Naval Observatory.
 
In astronomy, the solstice occurs at the moment the earth's tilt away from the sun is at a maximum. Therefore, on the day of the winter solstice, the sun appears at its lowest elevation with a noontime position that changes very little for several days before and after the winter solstice. In fact, the word solstice comes from Latin solstitium or sol (the sun) + -stit-, -stes (standing).

A pinhole camera (no lens and a single small aperature) can be effectively used to document the change in elevation of the sun during the year.  The image to the right is a solargraph, in which the path of the sun as it crosses the sky is captured for an extended period.  It was made using a pinhole camera and depicts the change in the elevation of the sun from the Autumnal Equinox (highest streak) to the Winter Solstice (lowest streak) at Flagstaff, Arizona.

Another solargraph made with a pinhole camera in Moriarty for six months in late 2009 is available here.

  

Solargraph for august through december

Becky Ramotowski/Brian Skiff
 
The Relationship between Length of Day and Temperature
 

It is interesting to note that the shortest day of the year (the winter solstice) is not normally the coldest day of the year. There is actually a lag between the shortest day of the year and the coldest average temperatures not only across New Mexico but for most of the mid and high latitude locations. In the graph at the end of this section, the length of daylight is plotted in red while the average daily temperature is plotted in blue, and the date of the solstice is depicted by yellow shading. The sun angle is low before and after the winter solstice with a minimum number of daylight minutes. As the sun climbs higher in the sky the length of daylight increases. In Albuquerque, the minimum daily temperature occurs around 2 weeks later, in early January. This lag in temperature occurs because even though the minutes of daylight are increasing, the earth's surface continues to lose more energy than it receives from the sun. Average temperatures continue to drop until the sun climbs higher in the sky. While the effect is evident in the daily temperature plot, it is more readily apparent by looking at changes in the monthly average temperature (below and to the right). In Albuquerque, January averages only 0.1F degrees higher than December, even though the days are becoming significantly longer. 

 
Another interesting fact depicted in the graph below is that while the solstice is the shortest day of the year, with 9 hours and 48 minutes of daylight in Albuquerque, it is just seconds shorter than days on either side of the solstice. In fact, the U.S. Observatory lists the length of daylight in Albuquerque (in 2011) as 9 hours and 47 or 48 minutes from December 16th through December 28th.

While locations in the Northern Hemisphere are experiencing their shortest day (and longest night) on the day of the Winter Solstice, the air above the South Pole is receiving more sunlight than any other location on the globe, but the temperature remains around -10° F!
 
The Solstices, Equinoxes and Seasons


We all know that the Earth makes a complete revolution around the sun once every 365 days, following an orbit that is elliptical in shape.  This means that the distance between the Earth and Sun, which is 93 million miles on average, varies throughout the year.  During the first week in January, the Earth is about 1.6 million miles closer to the sun. This is referred to as the perihelion.  The aphelion, or the point at which the Earth is about 1.6 million miles farther away from the sun, occurs during the first week in July.  This fact may sound counter to what we know about seasons in the Northern Hemisphere, but actually the difference is not significant in terms of climate and is NOT the reason why we have seasons.  Seasons are caused by the fact that the Earth is tilted on its axis by 23.5°.  The tilt's orientation with respect to space does not change during the year; thus, the Northen Hemisphere is tilted toward the sun in June and away from the sun in December, as illustrated in the graphic below. 

The summer solstice occurs when the sun is directly over the Tropic of Cancer, which is located at 23.5° North, and runs through Mexico, the Bahamas, Egypt, Saudi Arabia, India, and southern China.  Because of the Earth's tilt, the sun is directly over the Tropic of Cancer near June 21. For every place north of the Tropic of Cancer, the sun is at its highest point in the sky and this is the longest day of the year. As stated above, the winter solstice occurs when the sun is directly over the Tropic of Capricorn, which is located at 23.5° south of the equator.

There are two times of the year when the Earth's axis is tilted neither toward nor away from the sun, resulting in an equal amount of daylight and darkness at all latitudes.  These events are referred to as equinoxes and occur near March 21 (Vernal Equinox) and near September 22 (Autumnal Equinox).  At the equator, the sun is directly overhead at noon on the two equinoxes.
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