Winter starts when?

I know what you're thinking: easy question. Ten seconds with Google will inform you that this year winter begins on Dec. 22 at 12:08am CST. So here are some tougher questions: For us northern hemisphere dwellers, on what day does the sun rise the latest? On what day does it set the earliest? Which day is the shortest? If you answered Dec. 22 for all of these questions, then you're 1 for 3. The shortest day of the year is indeed the winter solstice (Dec. 22), but the earliest sunset is almost three weeks earlier on Dec. 5. And the latest sunrise won't occur until Jan. 6! So why are these three dates spread out over a whole month, rather than all on the same day, as intuition might suggest? To answer that, we'll have to take a step back and look at the big picture - from space.

The answer lies in two aspects of the earth's orbit around the sun. Firstly, our earth is tilted with respect to its orbit at an angle of 23.5 degrees. Without this axial tilt, there would be no seasons. More importantly to the discussion at hand, this tilt causes the sun to change its elevation in the sky - the sun is much higher at noon in the summer than in the winter. Secondly, the earth's orbit is not a perfect circle, but ever so slightly elliptical (the eccentricity of our orbit is 0.017). Because of this, we are a few million kilometers closer to the sun at some times of the year than at others. Kepler's second law of planetary motion states that a planet moves faster when it is closer to the sun - so the speed at which the earth moves around the sun varies slightly throughout the year.

Because of these two variations, the length of a "solar day" (the time it takes for the sun to return to the same longitude) changes as the earth orbits the sun. The solar day is shortest in March and September, and longest in June and December. This change is often visualized using the analemma - the figure-8 motion that the sun makes over the course of a year as viewed from Earth. This is pictured above: the motion up and down the figure-8 is a result of the tilt in the earth's axis, while the side-to-side component of the figure-8 results from the elliptical nature of the earth's orbit. These effects combine to move the sun around the figure-8 of the analemma over the course of the year in the direction shown by the arrows in the diagram.

As expected, the shortest day of the year occurs at the bottom of the analemma, on Dec. 22. At that point, the sun spends the least time above the horizon. To think about the latest sunrise, look at the analemma diagram above, which was taken in the morning. The latest sunrise occurs when the sun is lowest on this diagram. As you can see, this occurs not at the "bottom" of the figure-8, but slightly to the left. Thus the latest sunrise occurs a couple weeks after the shortest day. At sunset, the analemma is tilted the opposite direction, so that the lowest point on the analemma is not at the bottom of the figure-8, but slightly to the right. Thus the earliest sunset occurs a couple weeks before the winter solstice.

So now you know why the shortest day, earliest sunset, and latest sunrise of the year don't all fall on the winter solstice. But before I end this lesson in climate and astronomy, let me dispense one more piece of trivia to impress your friends with: The earth is actually closest to the sun in January, not in June as many people would expect! Because of this, our winters here in the northern hemisphere are just a little bit warmer and our summers just a little bit cooler than they would be if our orbit was circular. Remember that small blessing when you're huddled under your coat to keep out the cold December wind!

For a more detailed explanation of the analemma and its effects on our calendar, check out this excellent explanation by John Holtz.



Story is © Nate Snook, 2007