diamond geezer

 Wednesday, March 21, 2007

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.

Time of Spring Equinox (GMT)
199208:48199314:41199420:28199502:15
199608:03199713:55199819:55199901:46
200007:35200113:14200219:16200301:00
200406:49200512:33200618:26200700:07
200805:48200911:44201017:32201123:21
201205:14201311:02201416:57201522:45
201604:30201710:28201816:15201921:58
March 20th (blue)   March 21st (green)

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]

• Spring equinox: what it is and why it moves
• Spring equinox: all the times and dates over 4 centuries
• Spring equinox: nice graphs to show how the date and time change


<< click for Newer posts

click for Older Posts >>


click to return to the main page


...or read more in my monthly archives
Jan17  Feb17  Mar17  Apr17  May17  Jun17  Jul17  Aug17  Sep17
Jan16  Feb16  Mar16  Apr16  May16  Jun16  Jul16  Aug16  Sep16  Oct16  Nov16  Dec16
Jan15  Feb15  Mar15  Apr15  May15  Jun15  Jul15  Aug15  Sep15  Oct15  Nov15  Dec15
Jan14  Feb14  Mar14  Apr14  May14  Jun14  Jul14  Aug14  Sep14  Oct14  Nov14  Dec14
Jan13  Feb13  Mar13  Apr13  May13  Jun13  Jul13  Aug13  Sep13  Oct13  Nov13  Dec13
Jan12  Feb12  Mar12  Apr12  May12  Jun12  Jul12  Aug12  Sep12  Oct12  Nov12  Dec12
Jan11  Feb11  Mar11  Apr11  May11  Jun11  Jul11  Aug11  Sep11  Oct11  Nov11  Dec11
Jan10  Feb10  Mar10  Apr10  May10  Jun10  Jul10  Aug10  Sep10  Oct10  Nov10  Dec10 
Jan09  Feb09  Mar09  Apr09  May09  Jun09  Jul09  Aug09  Sep09  Oct09  Nov09  Dec09
Jan08  Feb08  Mar08  Apr08  May08  Jun08  Jul08  Aug08  Sep08  Oct08  Nov08  Dec08
Jan07  Feb07  Mar07  Apr07  May07  Jun07  Jul07  Aug07  Sep07  Oct07  Nov07  Dec07
Jan06  Feb06  Mar06  Apr06  May06  Jun06  Jul06  Aug06  Sep06  Oct06  Nov06  Dec06
Jan05  Feb05  Mar05  Apr05  May05  Jun05  Jul05  Aug05  Sep05  Oct05  Nov05  Dec05
Jan04  Feb04  Mar04  Apr04  May04  Jun04  Jul04  Aug04  Sep04  Oct04  Nov04  Dec04
Jan03  Feb03  Mar03  Apr03  May03  Jun03  Jul03  Aug03  Sep03  Oct03  Nov03  Dec03
 Jan02  Feb02  Mar02  Apr02  May02  Jun02  Jul02 Aug02  Sep02  Oct02  Nov02  Dec02 

eXTReMe Tracker
jack of diamonds
Life viewed from London E3

» email me
» follow me on twitter
» follow the blog on Twitter
» follow the blog on RSS

my flickr photostream