Of the four seasonal markers, this one seems to be the least understood (and for many the least welcomed with winter now ahead of us!). Contributing to this is a widely believed fact that not only is counter-intuitive—but is actually wrong!
The Sun arrives at the winter solstice at 10:50 a.m. EST on December 21st (Actually it’s the Earth in its orbital motion that has reached this point—the Sun only appears to have moved there.) The south pole of the Earth will then be tilted towards the Sun (and the north pole away from it), which will then have reached its southernmost position directly over the Tropic of Capricorn at 24 degrees south latitude. This is the first day of winter in the Northern Hemisphere and the first day of summer in the Southern Hemisphere (their summer solstice). The word “solstice” essentially means “Sun stand still” which it appears to do at this time, ending its six-month southward journey from our summer solstice in June.
Most people believe that we are farthest from the Sun in December and closest to it in June. After all, it’s cold in December and warm in June—right? Wrong! At the winter solstice we are 91,400,000 miles from our Daytime Star and 94,500,000 miles from it at the summer solstice. So in winter we are actually 3,000,000 miles further away from the Sun. What’s wrong with this picture, as they say? It’s not the 3 percent difference in our distance from the Sun that causes the temperature difference of the seasons but rather the angle at which sunlight hits the Earth. In
winter being tilted away from the Sun, its light arrives at a glancing angle and its heat is therefore spread out over a greater surface area. In summer it arrived directly-on and therefore its heat is much more concentrated.
There’s also the issue of “seasonal lag” to consider. We all know that there is a delay in the temperature change at the solstices typically of several weeks. This involves absorption and radiation of the Earth’s heat reservoir and the atmosphere requiring time to adjust to the changing heating from the Sun (less in winter, more in summer). And you would think once the Sun reaches its lowest point in the sky and starts back northward that the times of sunrise would slowly get shorter and of sunset get longer. But again it takes a while to see this happen. And here it has to do with subtle aspects of the Earth’s elliptical orbit.
This is part of it: We are actually at our closest distance to the Sun (called perihelion) on January 2nd and not December 21st. Likewise, we are at our greatest distance (called aphelion) on July 5th and not on June 21st. But as far as the Sun’s apparent position in the sky, this has no impact! Wow. Did you ever think that the events marking the beginning of a season were so complex?
In closing, here’s something less cerebral. The annual Geminid meteor shower—one of the year’s best displays (considered by some superior to the famed Perseid shower in August)—occurs on the night of December 13th to 14th. Unfortunately, the Moon will be just a few days past Full on its way to Last Quarter and will light up the sky for much of the night.
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of 10 books on stargazing. His latest, Celebrating the Universe!, is available from HayHouse.com.