On March 20, as on every vernal equinox, schoolchildren learn and the media proclaim that day and night are equal everywhere. It’s an appealing thought. Our planet, flying through space in a Charlie Chaplin wobble, momentarily, at 4:29 P.M. eastern time, tilts neither toward nor away from the sun. In theory this balancing act bestows a universal equilibrium. And in fact it is a time of approximate equality in most of the world, a far howl from the long nights and short days we’ve endured the past several months. But the truth is both more complex and more interesting than the common wisdom.
The monkey wrench thrown into this worldwide equality business is our own atmosphere. Whenever a celestial body sits on the skyline, the horizon’s thick air bends its image upward by half a degree, the way a glass of water appears to bend a spoon. By sheer coincidence, this half- degree deflection exactly matches the apparent size of the moon or sun. So the entire sun is yanked upward a distance equal to its own diameter as it sets, producing a phantom image. Whenever the sun appears just above the horizon, you’re seeing a ghost, an illusion.
Add in the same effect at sunrise, and we get about ten minutes of extra sunshine on the day of the equinox. So the symmetry we’d experience on an airless planet like Mercury is replaced with inequality: day is favored over night. The real equilibrium date occurs earlier in the Northern Hemisphere (March 15 in Miami) and later throughout Earth’s southern half (March 26 in Rio). For the colonies of mad scientists doing research at the poles, reality is even further from equality. There, atmospheric refraction compels the sun to shine the entire equinox day. No night at all. No hint of balance.
This doesn’t mean the occasion is totally without a shared experience. The sun rises and sets precisely due east and west only on the equinox, offering the year’s best chance to get your bearings. After all, a compass merely follows the magnetic poles, which are not aligned with the true cardinal directions. In Boston, for example, a compass points a whopping 16 degrees west of north. But on March 20 we get the real thing. No matter where you live, the rising sun marks exact east, while sunset stamps true west with its fire.
Do you associate the equinox with March 21? Then you’re remembering the good old days of a vanished era. Almanacs using Greenwich time will list four final March 21 equinoxes during the next dozen years, but if you reckon in eastern time or any other time zone of the United States, we had our concluding March 21 equinox more than a decade ago. It will fall only on the nineteenth and the twentieth from now until the twenty-second century.
The equinox landed mostly on the twenty-first during the first half of this century. It’s a nearly unknown fact, but our calendar system causes equinoxes and solstices to creep progressively earlier. The lack of a leap year at century’s end usually restores the order, making most centuries comparable to the one before.
But not this time. The additional insertion of a February 29 in the year 2000 (a once-every-400-years calendrical fine-tuning) allows the regression to go unchecked. So forget the twenty-first and salute the sun on the twentieth. Even if (all things being equal) spring’s first day is as unbalanced as the federal budget.