Wednesday was the winter solstice, the shortest day of the year, when the midday sun barely clears the backyard treetops.
It was not, however, the day of the year’s earliest sunset. Or latest sunrise.
This seems counterintuitive. And it is, actually, the same way it’s counterintuitive to think the Earth goes around the sun — since the sun obviously rises, creeps across the sky and then sets every day. Astronomers spent thousands of years wondering why the math in their calculations of the sun and planets circling the Earth didn’t quite work. In the 1500s, Copernicus figured out appearances did not correspond to reality when he got better results calculating the orbits of the Earth and other planets circling the sun, instead.
The midmorning sun in December in Troy. Dana Wilde photo
If you stand back like Copernicus and picture a few details about the Earth’s orbit around the sun, you can get a grasp of why the shortest day does not have the earliest sunset.
First, picture in your mind the Earth spinning. It makes one complete spin in just about 24 hours, carrying us into the sun’s rays (day) and then out of the sun’s rays (night).
Now picture the Earth traveling around the sun while it spins. It takes just about 365¼ days — one year — to make one circuit. Now, as the Earth is spinning and revolving, picture it tilted by about 23½ degrees. The top (North Pole) is tilting toward the sun for half the orbit, then tilting away from the sun the other half. When the top is tilted toward the sun, the angle of the sun’s rays is higher, there’s more sunlight and days are longer. The sun appears highest in the sky around June 21, the summer solstice. When the top is tilted away from the sun, the angle of the sun’s rays is lower, there’s less sunlight and days are shorter. The sun’s lowest high point occurs around Dec. 21, the winter solstice. (All this occurs oppositely in the Southern Hemisphere.)
So the Earth is spinning, revolving and tilted.
Next, the Earth’s orbit around the sun is not a perfect circle. It’s an ellipse, a slightly flattened circle. The sun is not at the center of this elliptical orbit; it’s a bit off center. So as it orbits, the Earth is closer to the sun for part of the year, and farther away the rest of the year. As it moves closer to the sun, the Earth speeds up. As it moves farther away, it slows down.
The Earth is nearest the sun around the beginning of January every year. So in December, in the weeks before the solstice, the Earth is speeding up because it’s nearing the sun.
The sun already appears to be making a lower and lower angle in the sky each day because of the Earth’s tilt. So the tilt, which accounts for changes in the apparent angle of the sun’s rays day by day; the orbital motion itself; the increasing speed of the motion; and the narrowing distance from the sun are all changing together like a clockwork of gears. All the gears are adjusting in tandem with one another, according to laws of physics, simultaneously. It’s almost like it’s all vibrating together.
The sun appears to be getting lower in the midday sky, but because of these interconnecting vibrations of elliptical motion, increasing velocity, and tilt, the sun’s apparent motion is not happening at the steady rate you might assume it would. One of the ways this becomes apparent to us is in the changing times of sunrise and sunset.
In Augusta, the earliest sunsets this year occurred at about 3:59 p.m. from Dec. 7 to 12, according to the timeanddate.com website. The latest sunrises will be at about 7:14 a.m. from Dec. 26 to Jan. 9. On the solstice — Wednesday, Dec. 21 — sunrise was at about 7:11 a.m. and sunset was at about 4:02 p.m.
Other vibrations are happening, too, which don’t have much effect on times of sunrise or sunset. For example, you can picture the tilted, spinning Earth as an old-fashioned top spinning on a desktop. Since it’s slightly tilted, the top wobbles. One wobble takes takes about 26,000 years to complete. This wobble is called precession.
Another wobble, in the Earth’s orbit around the sun, is caused by the moon’s gravity. One wobble occurs every orbit of the moon around the Earth, about 27.3 days.
Another variable in the Earth’s motions is that its spin is overall slowing under the frictions of tides, which are caused mainly by the moon’s gravity. Over the very long term, this slowing will continue until the Earth’s rotational period is the same as the moon’s orbital period.
But those tidal frictions also change the Earth’s spin by milliseconds, faster or slower, from day to day. According to measurements by atomic clocks, Earth’s spin has been tending a tiny bit faster lately, creating shorter days. The shortest day ever recorded was June 29 this year, when Earth completed one spin in 24 hours minus 1.59 milliseconds.
Even in the sky, appearances may well not be reality. Just like in life.
Dana Wilde lives in Troy. You can contact him at dwilde.naturalist@gmail.com. His book “Winter: Notes and Numina from the Maine Woods” is available from North Country Press. Backyard Naturalist appears the second and fourth Thursdays each month.
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