National Weather Service United States Department of Commerce

The winter 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 solstice.  In fact, the word solstice comes from Latin solstitium or sol (the sun) + -stit-, -stes (standing). 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 9:19 pm MST on December 21, 2019. For a complete listing of the dates of the winter and summer solstice's and spring and fall equinox's, check out: https://www.weather.gov/media/ind/seasons.pdf

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 below is a solargraph made with a pinhole camera, in which the path of the sun as it crosses the sky is captured for an extended period. It was made in Tijeras, and depicts the period from the summer solstice (highest streaks) to the winter solstice (lowest streaks) in 2009.   

 Image courtesy of Becky Ramotowski
The Relationship Between Length of Day and Temperature

Some of you may be wondering why the shortest day of the year is not normally the coldest day of the year.  There is 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 blue while the average daily temperature is plotted in red, 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 begins to move higher in the sky the length of daylight increases.

In Albuquerque, the minimum daily temperature occurs for about 2 weeks after into early January. This lag in temperature occurs because even though the minutes of daylight are increasing, the earth's surface and atmosphere continues to receive less energy than just what it receives from the sun.  Average temperatures continue to decrease until the sun rises higher in the sky. The effect is evident in the daily temperature plot and also by looking at changes in the monthly average temperature (below and to the right). In Albuquerque, January averages only 0.1°F degrees higher than December, followed by a sharp rise in February.
 
Another interesting fact depicted in the graph below is that while the winter solstice is the shortest day of the year, with 9 hours and 47 minutes of daylight in Albuquerque, it is just several seconds shorter than days on either side of the solstice.  In fact, the U.S. Observatory lists the length of daylight in Albuquerque as 9 hours and 50 minutes or less from December 12th through December 31st.

 

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°.  This can be seen graphically in the picture below.


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 21st (Vernal Equinox) and near September 22nd (Autumnal Equinox).  At the equator, the sun is directly overhead at noon on the two equinoxes.