The winter solstice, marking the moment that the season of fall ends and winter begins in the northern hemisphere, took place at 5:04 a.m. Sunday. The winter solstice is a very happy day and a very sad day – literally SAD. Here’s why:

If the Earth sat upright on its rotational axis, the sun would simply hover over the Earth’s equator all year long and we would not experience any seasonal changes. Instead, the 23.5-degree tilt of the Earth on its axis causes the sun to wander 23.5 degrees north of the equator and 23.5 degrees south of the equator. The dates of the two extremes are called the solstices. Solstice is a word that literally means, “the sun stands still.”

The farther south the sun moves, the fewer hours of sunlight we enjoy each day. As we approach the date of the winter solstice, our days become shorter and shorter as the sun heads south. If it kept going, we eventually would be cast into eternal night. It is a happy day indeed when the sun bottoms out and begins to head north again. The ancient civilizations broke into wild celebration when the sun stopped heading south and “stood still” for a day. The Romans used to pull out all of the plugs and throw a huge party called “Saturnalia.” They celebrated the “birthday of the invincible sun.”

With each successive day after the winter solstice, the daylight hours grow longer and the promise of spring is in the frosty air. Unfortunately, the news is not all good for some folks who suffer from SAD, or Seasonal Affective Disorder. The waning sunlight can cause some people to sink into the deep depression of SAD.

For the two winters that I lived in Anchorage, Alaska, I felt the sting of SAD a little bit myself when the sun came up at 11 a.m. in the dead of winter and set only four hours later at 3 p.m. The therapy for SAD is to sit for extended periods of time under bright artificial lighting. This seems to satisfy the body’s need for sunlight and can pull SAD sufferers out of their deep, dark hole.

Evergreen trees have become a part of our seasonal celebrations because they symbolize the triumph of life against the cold and dark of winter. During the last few dark and dreary days near the solstice, it helps to remember that while we look forward to three months of winter, folks down under are doing just the opposite – looking forward to three months of summer.

Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. He is an avid astronomer whose photographs and articles have been published in a variety of magazines and Web sites.