Monday 20 brings the spring equinox. If this date surprises you because you associate vernal equinoxes as occurring on the 21st, you’re dating yourself.
Spring arrived on the 21st during most of the 20th century. They slide earlier and earlier during the 400-year Gregorian calendar cycle. The final March 21 equinox was in 2007, even if we use Greenwich Time as many almanacs do. Now the 21st is gone for keeps, unless you believe in reincarnation and want to check back in the 22nd century. In a few more decades, the vernal equinox will sometimes start landing on the 19th.
The equinoctial sun always misses your zenith or straight overhead point by the same number of degrees as your latitude. In Bennington, Vermont, latitude 43°, Monday’s midday sun stands 43° from precisely overhead. Essentially it’s halfway up the sky. It’s a gratifying change from just a month ago, and a dramatic shift since December, when the midday sun only climbed an anemic one-fifth of the way up the sky. Since solar rays are stronger the higher up it is, you can now palpable FEEL the sun’s growing intensity.
And it’s not finished: Watch the sky at 1:00 PM each day, and you’ll see that the sun manages to climb four of its own diameters higher each week. This rapidly ratchets up its intensity. This is the year’s greatest solar-energy boost for those who live north of the equator.
Spring Equinox Fun Facts
- Equal day/night: The equinox is famously the time of balance, with theoretically 12 hours of sunshine and 12 hours of non-sun. In practice, the atmosphere bends the sun upward so that the real date of sunlight equality is three or four days ahead of the equinox, meaning before this weekend!
- East/West phenomenon: The equinox is also when every place on earth rotates perpendicularly into our planet’s —terminatorits day-night shadow line. As a result, on Monday the sun will rise precisely due east and set exactly in the west, and this is true everywhere. It’s the best time to observe the cardinal compass directions.
Well, not quite everywhere. From both poles, you’d see Monday’s equinoctial Sun hovering fully above the horizon, never setting, but moving horizontally. At the north pole the Sun moves rightward, and it chugs along leftward for the folks at the South Pole research station, rolling atop the horizon like a red rubber ball.
- Sunlight changes: This is also the week when sunlight changes at its maximum annual rate, with three extra minutes of daily Sun from typical US cities, but nearly seven daily minutes for the folks in Fairbanks, Alaska. That may be the equinox’s greatest gift, and worthy of an early morning Sun Salutation.