My father, who notices these things, once noticed that the newspapers seemed to be misprinting the times of sunrise and sunset in December. They said the sun wouldn’t start rising earlier until a full 10 days after the winter solstice, which is the shortest day of the year. Afternoons were even stranger: the newspapers said they would start lengthening 10 days before the solstice.
When he soon discovered that the newspapers weren’t messing it up at all – that the earliest sunset and latest sunrise do indeed offset the solstice by about 10 days – he launched an impromptu series of after-dinner discussions in which he, I, and anyone else at hand would deploy salt shakers and all manner of condiments, trying to align the Earth and sun in some kind of configuration that could account for this observed phenomenon.
After several years, dozens of dinners, and countless regrettable theories (this being before you could look up these sorts of things on the Internet), we were nowhere. That’s when my father stumbled across a book entitled, humiliatingly enough, The Beginner’s Guide to the Skies.
It seemed that we had been focusing on the wrong time of day. It wasn’t sunrise and sunset that were the problem, it was noon.
Those of you still laboring under the misconception that noon is noon are in for something of a shock. There’s clock noon, the 24-hour affair that we track with our watches. And then there’s solar noon – that moment of each day when the sun is directly overhead.
If the Earth’s orbit were a perfect circle, and if its axis weren’t tilted, clock noon and solar noon would be one and the same. Every time your watch read noon, the sun would be directly overhead.
In fact, the Earth’s orbit is an ellipse, and geometry requires that the Earth move faster along its orbit when it is close to the sun and slower when it is far away. This causes clock noon and solar noon to get out of synch. When the Earth is speeding along close to the sun, it essentially leaves the sun behind; the clock says noon, but the sun has not quite returned overhead. The Earth has to turn for some extra seconds to reach solar noon, and the solar day winds up longer than the clock day.
The situation is reversed near the summer solstice, when the Earth is far from the sun and moving relatively slowly. Then, the sun is overhead before clock noon, making the solar day shorter than the clock day. Over the course of an entire year, the lengths of the solar days average out to a clock day, which is, of course, the very definition of a clock day.
There is a lesser factor at work here, too, which is the tilt of the Earth’s axis. As my college professors used to say, this is “beyond the scope of this course.” If you want to satisfy your curiosity in more detail, I’d recommend taking a look at The Beginner’s Guide to the Skies (assuming your self esteem will allow it). Otherwise, conduct an Internet search on the “equation of time,” which is how astronomers combine these two factors to accurately predict the timing of sunrise and sunset. The important take-home summary is that a combination of the Earth’s elliptical orbit and tilted axis causes solar noon to vary over the course of the year compared with clock noon.
Now, back to the original question: why don’t the latest sunrise and earliest sunset fall on the winter solstice? In early December, clock noon is about 15 minutes behind solar noon. By late January, clock noon has jumped to 15 minutes ahead of solar noon. Throughout December and January, therefore, roughly 30 minutes of daylight shift from morning to afternoon, averaging about 30 seconds each and every day.
On the 10 days either side of the winter solstice, this “noon correction” is closer to a full minute and is greater than the change in day length itself (which is very small close to the solstice). In other words, for 10 days before the solstice, daylight is being added to the afternoon faster than it is being lost, causing the sun to set later even while the day itself is growing sorter. Similarly, for 10 days after the solstice, daylight is being taken from the morning faster than it is being created, causing the sun to continue rising later even though the days are growing longer.
This “morning/afternoon” correction is only half as great at the summer solstice and is not enough to overcome the change in day length, which is why the longest summer morning and longest summer afternoon both fall on the solstice itself, as would be expected.
As my Dad would expect, anyway.
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