It's 9:41 a.m. March 20. Do you know where your planet is?
If your answer is, "At the vernal equinox," Leroy E. Doggett says you're only part right.
Raise your hand and shout, "The first day of spring!" and you also win just partial credit from Mr. Doggett, who is chief of the nautical almanac office at the U.S. Naval Observatory in Washington.
The fact is, this equinox business is no simple matter. Nothing is sacred, not even the notion that today marks the first day of spring. In spite of last week's blizzard, for example, the ancient Celts might argue that today marks the middle of spring.
But let's start with the basics.
"We define the vernal equinox to be when the . . . Earth has dTC reached a specific point in its orbit around the sun," Mr. Doggett said. "The way we determine that point becomes very complex. . . . We begin with Isaac Newton and incorporate Einstein's relativity theory."
Stir in four or five parts direct daily observation of the sun, and . . . well, you get the idea.
"Nobody legislates this," Mr. Doggett said. "Certainly the U.N. isn't capable of it, and the pope isn't interested."
Setting the precise date and time of the equinox requires the cooperative efforts of the U.S. Naval Observatory; the Royal Greenwich Observatory in England; the Bureau de Longitude in Paris; the Institute for Theoretical Astronomy in St. Petersburg, Russia; and the Astronomisches Rechen Institut in Germany.
Each year, they publish the next year's seasonal data in the Astronomical Almanac. They could run the calculations out many years in advance. But if they did, Mr. Doggett said, the irregular slowing of the planet's rotation would throw off the seasons' arrivals by perhaps a minute every 30 years.
Also, today is the vernal equinox only in the Northern Hemisphere. South of the equator, it's the autumnal equinox. Astronomers try to avoid hemispheric chauvinism by sticking to just plain "equinox."
Ancient observers might say the equinox is the day that the sun rises at due east, or halfway between its southernmost sunrise (or winter solstice) and its northernmost sunrise (summer solstice).
Modern astronomers say the equinoxes occur when the center of the Earth passes through two theoretical points on opposite sides of the planet's near-circular orbit. Those spots are the places where the Earth's orbital plane (called the plane of the ecliptic) is intersected by a second plane, called the celestial equator.
The celestial equator is aligned with the Earth's equator, sort of like the rings around Saturn, only extended invisibly and infinitely out into space. Because the Earth is tilted 23 1/2 degrees from the plane of the ecliptic, the two planes intersect only along a line across the solar system from one one side of the Earth's orbit, through the sun to the opposite side of the orbit, touching that orbit at the equinoxes.
Well, just think of the equinoxes as the orbital mid-points between the winter solstice (the shortest day, when the Earth's Northern Hemisphere is tilted farthest away from the sun, and temperatures are cool) and the summer solstice (the longest day, when the Northern Hemisphere is tilted toward the sun and temperatures are warm). Day and night at the equinoxes are about the same length - hence the name, from the Latin aequus (equal) and nox (night).
And none of it, except for tradition, has much to do with the start of spring or autumn weather. Around these parts, the vernal equinox can be balmy or snowy.
"You have the same problem with the other seasons," said Mr. Doggett. Winter weather often arrives in "autumn," and summer heat can start in spring.
The ancient Celts of western Europe, like many other ancient peoples, would agree.
Generations of careful solar observations became an essential guide for their agricultural and spiritual lives. And in their reckoning, the year was divided not into quarters, but into eighths. In addition to the solstices and equinoxes, they observed four "cross-quarter days."
"A cross-quarter day is a mid-point between a solstice and an equinox," Mr. Doggett said, and it was those days that marked the start of the seasons as we think of them.
"Groundhog Day is a descendant of one of these," he said. Now observed Feb. 2, Groundhog Day was once the Christian Candlemas, but before that it was the Celtic festival of Imbolg. "It was at that time of the year when sheep should begin having milk, and Imbolg itself means sheep's milk," he said. It was, in a sense, the real beginning of spring, anticipating the birth of lambs.
The next cross-quarter day was May Day, or Beltaine. "It was associated with a pastoral god concerned with sun worship," Mr. Doggett said, and it marked the time for driving cattle to summer pastures.
If May Day was in that sense the first day of summer, it explains why the summer solstice around June 21 was, and is still, called Midsummer Day in many cultures, and not the first day of summer.
To the Celts, the next cross-quarter day was around Aug. 1. Called Lughnasa, it was a festival of "a rather mischievous god named Lugh, who was associated with early harvest," Mr.
Doggett said. In that sense, it marked the start of the harvest season, which we associate with autumn.
The final cross-quarter day in the Celtic year was Samhain, which fell around Nov. 1. Christians later appropriated the date as All Saints Day. But to the Celts, Mr. Doggett said, it was associated with malevolent forces, death and the coming of winter. Aspects of it have been preserved in our Oct. 31 Halloween traditions.
The winter solstice, which we regard as the start of winter, was for the Celts a time for mid-winter rituals to ensure the rebirth of the sun. If the rituals worked, the days would thereafter grow longer and winter would be on the wane.
And they always worked.