Solstice vs. Equinox

When it comes to the workings of the universe, Albert Einstein famously said, “God does not play dice.” Einstein said nothing about the dreidel, however, which may explain why Earth and the other planets in our solar system spin like tops, a touch wobbly but more or less self-​correcting, whirling about in space toward whatever bang or whimper the future has in store.

Draw a line through our planet from pole to pole and tilt Earth at an angle of 23.4°, and you have what scientists call the “obliquity of the ecliptic.” Earth spins along, maintaining that angle relative to the sun, so that, for roughly half the year, the Northern Hemisphere is bathed in the light of Sol, while for the other half, it tilts away from the sun.

From our point of view, the sun appears to move in the sky from north to south relative to the equator. On or around March 20, it ascends north until, on or around June 21, it reaches its apex at the Tropic of Cancer. The first date is the “vernal equinox,” when, the sun being at the equator, day and night—​nox, in Latin—​are roughly equal in duration. The second is the “summer solstice,” the latter word coming from the Latin sol, “sun,” and sistere, “to stand still”: Thus the sun appears to hang in the sky around the clock, the source of the endless daylight of summer high up in the northern latitudes, the fuel for Erik Skjoldbjærg’s cold-​sweat-​inducing film Insomnia.

Come the end of summer—​on or around September 23—​the sun clambers down the sky to the equator and the autumnal equinox, then below the equator to the winter solstice of December 21, when the length of the day in the Northern Hemisphere is the shortest of the year. Thus solstices and equinoxes, and thus the origins of the seasons.

Over the course of millennia, Earth travels in a precession, the wobble of the gyroscope altering its position relative to the stars so that, over a nearly 26,000-​year cycle, Earth’s north pole points to different places in the sky, which yields minor variations in just when those seasons begin. When the Egyptians built the pyramids, which may have served in part as astronomical observatories, the north star was Thuban, in the constellation Draco, about halfway between the bend of the Big Dipper’s handle and the Little Dipper’s bowl. Today the north star is bright Polaris, in Ursa Minor. Earth spun its closest path ever to Polaris just a couple of years ago; now we’re bound for another north star, Gamma Cephei, which will take over the job from Polaris in about 2,000 years.

The laws of astronomy hold constant, then, while the objects of astrology—​the prescientific study of the night sky, full of fond hopes and dark premonitions—​are always unfixed. To name just one example, owing to the workings of the precession, thirteen constellations, not twelve, now pass overhead. That jumbles the traditional astrological calendar so much that even a true believer will want to consider the horoscope a suggestion rather than a mandate, a pastime and not a prophecy.