The last first day of Spring(vernal equinox 00:07 GMT, 21/03/07)
Whatever our topsy-turvy weather might suggest, Spring really is getting earlier. And this isn't the result of global warming, frost-free winters or extended growing seasons - this is astronomical.
The spring equinox occurs today, at seven minutes past midnight, making today the official first day of Spring. You might think that March 21st is always the first day of Spring but, astronomically speaking, this isn't the case. In fact, contrary to what you might expect, today is the very last day in your lifetime that the first day of Spring will fall on March 21st. Make the most of it.
The science: The spring equinox occurs when the shadow of the Sun crosses the equator, from the southern to the northern hemisphere, and its precise time varies from year to year. It takes the Earth approximately 365 days, 5 hours and 49 minutes to orbit the Sun, so every year the spring equinox shifts almost 6 hours later than the year before. That's approximately 24 hours later every 4 years, which is then cancelled out by the presence of a February 29th a few weeks before the next spring equinox occurs. But this still leaves the calendar 11 minutes short of reality every year, and this tiny difference shifts the spring equinox approximately three-quarters of an hour earlier every 4 years.
You can see from the table below how both the time and the date of the spring equinox change over a 28-year period. Look across the rows to see how the times jump approximately six hours later each year. Look at the final column in each row to see that the latest equinoxes always occur in years immediately before a leap year. And look down the columns to see how the times nudge about 45 minutes earlier every 4 years. That's how 2007 gets to be the very last occasion this century that the spring equinox will occur on March 21st. Next year, and for the foreseeable future, Spring will commence on March 20th instead. It won't be until 2102 before the equinox finally returns again to March 21st, and you'll almost certainly not be alive to see it.
In the year 2044 the times in the first column of this table will retreat past midnight, and this will push the spring equinox back one further day onto 19th March (initially for leap years only). From 2008 to 2043, however, the spring equinox will only ever be on 20th March. That's the unique date for the first day of Spring for the next four decades - so you'd better get used to it. From next year Spring really is starting earlier - it's official.
More science: If a year divides by 100 but not by 400, then it's not a leap year. So 2000 was a leap year, but 1800 and 1900 weren't, and 2100 won't be. This centennial jolt is designed to keep the seasons in track. It stops the spring equinox from creeping earlier and earlier across the millennia, and sets up a pattern that repeats (near enough) every 400 years. It also keeps the first day of Spring tethered within the period from 19th to 21st March, come what may. [N.B. this is only true for the UK and other countries in the GMT timezone. In Australia and New Zealand this equinox occasionally sneaks onto 22nd March, and in America onto 18th March]
Possible dates of the spring equinox (GMT) 1800-1875: 20th March or 21st March 1876-1899: 20th March only 1900-1911: 21st March only 1912-2007: 20th March or 21st March 2008-2043: 20th March only 2044-2099: 19th March or 20th March 2100-2135: 20th March or 21st March 2136-2175: 20th March only 2176-2199: 19th March or 20th March [cycle repeats every 400 years, approximately]