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.
