The winter solstice coinciding with the conjunction of Jupiter and Saturn has had me thinking about the music of the spheres*—the sublime harmony of celestial motion. Jupiter completes its path around the sun—perceived by us as a march through the Zodiac—every twelve years. It takes Saturn 29 years to complete the same circuit. Which means that Jupiter catches up with and passes Saturn about every seventeen (~12x[1+12/29) years. Because the orbits of the two planets are tilted with respect to one another they don’t usually get all that close when Jupiter overtakes Saturn. But this year the overtaking occurs when their respective orbital planes cross, so they get really close. As you’ve probably read, they last got this close around 800 years ago. Or was it 400? No matter—long before any of us was born.
And then there’s the winter solstice. As everyone knows, Northern Hemisphere days get shorter as we approach the solstice and start to get longer after the solstice—December 21 this year (and most years). Less obviously, sunsets get later over the couple of weeks prior to the solstice even though the days are getting shorter. This is because of Earth’s elliptical orbit and the fact that the solstices occur at the long ends of the ellipse. The laws of motion dictate that the earth moves faster on an angular basis relative to the Sun when it’s at either end of the ellipse. This effect is stronger for the winter solstice than the summer solstice because the former happens to occur when the earth is at the end of the ellipse that’s closest to the sun—”perihelion.” The earth is getting ahead of the sun, so both sunrise and sunset get later each day before the solstice even though the days are getting shorter. All this stuff is kind of hard one’s head around, unless you’re Newton or Keppler, but that’s okay. Just as you don’t need to know a D-minor from an arioso to appreciate Beethoven you don’t need to know physics to appreciate the music of the spheres**. (But it helps.)
*Wikipedia describes musica universalis, also called music of the spheres, as “an ancient philosophical concept that regards proportions in the movements of celestial bodies—the Sun, Moon, and planets—as a form of music.” It further says that the term “music” is not literal, but rather a “harmonic, mathematical, or religious concept.” Check out the Wikipedia entry for “musica universalis.”
**There’s a medium-technical explanation of all this here: https://earthsky.org/earth/winter-solstice-and-late-sunrise