Some of us agonise over whether spring begins with March or February. For Americans and astronomers, however, it starts at the vernal equinox, when the sun on its seasonal migration to facilitate our northern summer reaches a point directly over the equator, and when day and night are of equal length throughout the world.
For the Americans, all is very simple. Like those whose task it is to calculate the date of Easter, they take the equinox as occurring every year on March 21st. Rural USA, therefore, will celebrate the birth of spring tomorrow with all the razzmatazz that only it can orchestrate. Participants will enliven the proceedings by demonstrating the apparent truth of one of their most cherished myths - that at the vernal equinox, and only on that day, is it possible to make a raw egg balance on its end.
In reality, of course, you can do this any day you wish, if only you try hard enough; there are no vernal astronomical conjunctions that might make it easier.
The start of astronomical spring, however, is a variable feast, since the vernal equinox does not always land on March 21st, but may occur as much as two days previously. Its apparent oscillation can be traced to our habit of inserting an extra day into the calendar every now and then. Had this not been a leap year, for example, today would have been March 21st and the equinox, which occurs today, would be on schedule. But because we had a February 29th, the equinox occurs on March 20th.
My sources tell me that the earliest vernal equinox in the current millennium will occur at 12.28 GMT on March 19th, 2496. The latest in the last 500 years was at 20.42 GMT on March 21st, 1503. And by the time you read these lines, this year's vernal equinox will probably be over: it occurred at 7.35 GMT this morning, March 20th.
Traditionally, Irish weather around the vernal equinox is expected to be boisterous, but early 19th century Irish scientist Richard Kirwan found in it a clue to summer.
"When there has been no particular storm about the time of the spring equinox," he wrote, "if a storm arise from the east on or before that day, or if a storm from any point of the compass arise near a week after the equinox, then in either of these cases, the succeeding summer is generally dry; but if a storm arise from the south-west or west-south-west on or just before the spring equinox, then the summer following is generally wet, five times out of six."