The Tilted Earth at Its ‘Equal Night of Spring’

A couple of weeks ago, I checked a wall calendar and noted that this year the vernal equinox falls on Tuesday, March 20, happily the publication date of this column. Not long afterward, however, my eye chanced upon another calendrical reference to the equinox — March 21.

I quickly consulted every one of my household’s 13 calendars: 6 put the equinox on March 20, 6 on March 21. As for the potential tie-breaker, my daughter’s “Star Trek” calendar, it had nothing to say about a geo-fixated occasion like the equinox, though it did point out that March 22 is the birthday of William “Captain Kirk” Shatner.

Now, we’re all accustomed to the general shiftiness of the four seasons, and the fact that their start dates in the third week of March, June, September and December may move a day or two from one year to the next. This is to be expected from our attempts to synchronize our rigid annual schedules with the gravitational nuances of Earth’s transit around the Sun.

But as I soon discovered in my attempt to resolve the calendar crisis, the vernal equinox in 2007 has the added snag of arriving at the querulous hour of just seven minutes past midnight, universal time, on March 21. Coordinated Universal Time is what used to be called Greenwich Mean Time, but the new name doesn’t make it any more universal than it ever was, and it remains a time zone centered in Britain. For those of us in the United States, the vernal equinox arrives while it is still the evening of March 20.

Whatever the date, go on and celebrate, for the vernal equinox is a momentous poem among moments, overspilling its borders like the swelling of sunlight it heralds. As with everything else about the seasons, the equinox is the result of Earth’s sizable tilt, 23.5 degrees relative to the plane of the orbit. That tilt is fairly fixed, and as Earth makes its way on its circumsolar migration and rotates on its imaginary skewer, the northern tip of the skewer always points toward the same spot in space, the bold sparkle of Polaris, the North Star.

Sometimes the northern skewer tip happens to be facing the Sun, and the northern hemisphere is bathed in the direct sunbeams and generous long days of summer, while the southern hemisphere receives only indirect lighting and hence calls the time winter. Six months later, the scene is reversed, with the northern axis tilted away from the Sun, and its hemisphere left to make do with the Sun’s cool, oblique glances.

Twice a year the axial skewer tips are pointing neither toward nor away from the Sun, but instead are positioned exactly off to the side. If you could imagine being on the Sun and looking out toward our imaginary-skewered Earth, it would be like looking at a ball of yarn with a knitting needle stuck through it, in perfect profile. These are the times of the equinox, when the linked geometry of Earth’s rotational and orbital planes together bestow a day of equal parts light and night across the entire globe. And while the equinox is formally calculated based on the moment when Earth first enters its profile position, the Sun is so comparatively huge that it takes us time to pass any point of it, and equinoctial conditions will effectively persist for several days.

Vernal equinox, the lovely little Latinate term that means “equal night of spring,” is used to indicate the March-based equinox even in the southern hemisphere, where the event is really the start of autumn. “It’s a very Northern-centric view of things,” said Dr. William Blair, an astronomer at Johns Hopkins University. “Then again, most of Earth’s land mass is in the Northern Hemisphere, so what are you going to do?”

For many cultures throughout history, people got ready to start tilling the fields. Ancient peoples may not have understood orbital mechanics, but they were tireless observers of the Sun, stars and planets and noted with enviable precision how the position of the sunrise shifted on the horizon throughout the year. By tracking solar motions, they kept track of time and could estimate with some security when the last frost had passed and it was safe to plant crops.

“It seems wonderfully appropriate that you would anchor the timing of your planting season directly to the source of it,” said Dr. Paul Doherty, a physicist and senior staff scientist at the Exploratorium museum in San Francisco.

Archaeological evidence abounds that astronomy is among the oldest of professions, and that people attended with particular zeal to the equinoxes and the solstices. The Great Sphinx of Egypt, for example, built some 4,500 years ago, is positioned to face toward the rising sun on the vernal equinox.

In the 1,500-year-old Mayan city of Chichén Itzá, in Mexico’s Yucatán peninsula, the magnificent Kukulcán Pyramid practically slithers to life each spring equinox evening, as the waning sun casts a shadow along its steps of seven perfectly symmetrical isosceles triangles, a pattern suggesting the diamondback skin of a snake.

“The snake’s head points north, to the sacred part of the site, and the snake’s body represents a kind of umbilicus between sky and earth,” said Dr. Isabel Hawkins, an astronomer at the University of California, Berkeley, who has traveled to the site to witness the alignment, as do about 60,000 people from around the world every March.

“It’s a truly international celebration of the equinox,” she said. “It gives you goose bumps to be part of it, and to share with a bunch of strangers an intimate, primal sense of your connection to the bigger universe.”

In the West, the equinox is intimately fastened to the holiest of Christian holidays: Easter is timed to occur the first Sunday after the first full moon that follows the vernal equinox. “One of the main motivations for having astronomers working at the Vatican,” Dr. Hawkins said, “is that they wanted to know the precise date for Easter each year.”

Ecstatic, ecclesiastic, serpentine or Dionysian, the rebirth of the Earth offers a second chance to us all. Aren’t you glad you have two days to do it?

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The Tilted Earth at Its 'Equal Night of Spring' by Natalie Angier