diamond geezer

 Sunday, September 23, 2007

Autumn Equinox (10:51 BST)

Well known autumnal fact 1: There are four seasons in a year
Officially, astronomically, the seasons all start and finish at either a solstice or an equinox. And today is the autumnal equinox, which makes today the start of autumn. It's all because the earth is tilted on its axis, which means that different parts of the planet get more direct sunlight at different times of the year. Yes, obviously. Now tell us something we don't know...

Little-known autumnal fact 1: Autumn is shorter than summer
It's true. There may be four seasons in a year, but they're not all of equal length. It's 94 days from the summer solstice to the autumn equinox, making summer the longest of all the seasons. But it's only 90 days from the autumn equinox to the winter solstice, making autumn four days shorter. Winter's even shorter still, just 89 days, the shortest season of all. We may be entering the cooler, darker half of the year (between now and next March), but it's also the shorter half of the year. Here are the precise figures.

21 Mar 07
21 Jun 07
23 Sep 07
22 Dec 07
21 Jun 07
23 Sep 07
22 Dec 07
20 Mar 08
Duration93 days94 days90 days89 days

So why aren't the four seasons equal? It's because the earth's orbit around the sun isn't circular, it's an ellipse, and so some bits of the earth's orbit are nearer to the sun than others. Kepler's Second Law says that planets travel a bit faster when they're closest to the sun, and a bit slower when they're further away. The earth happens to be closest to the sun in early January, which is when it travels fastest, so autumn and winter are relatively short. And the earth is furthest from the sun in early July, which is when it travels slowest, so spring and summer are relatively long. It's not by much, but it's enough to make a difference. Autumn isn't a quarter of a year long, it's really 1½ days shorter than that. Make the most of it.

Well known autumnal fact 2: Today the sun is directly overhead at the equator
Sorry to those of you with Seasonal Affective Disorder, but nine minutes to eleven this morning is the official moment when the overhead sun switches from the northern to the southern hemisphere. It's about to be spring in the latter, but it's about to be autumn in the former. Damn. Or, if you're Antipodean, hurrah.

Little-known autumnal fact 2: Today is not the day that day and night are of equal length
It's true. Sunrise in London this morning was at 6:47am, while sunset tonight is at 6:58pm. So that makes the day 22 minutes longer than the night. Equality comes in three days time, around September 26th. Just look how rapidly the gap between daylight and darkness is changing this week. It's always fast-changing like this around the equinoxes.

DateSep 21Sep 22Sep 23Sep 24Sep 25Sep 26Sep 27
by 36min
by 28min
by 22min
by 12min
by 2min
by 12min

So why aren't day and night equal today? It's because the sun isn't a point of light as seen from the earth, it's a disc. At sunrise the top of the sun peeps above the horizon about a minute before the centre. And at sunset the top of the sun dips below the horizon about a minute after the centre. Atmospheric refraction means that we can still see the sun even when it's really below the horizon, adding a few more minutes to both of those times. And it's that pair of extra minutes that make all the difference. Daylight today is therefore still 22 minutes longer than darkness. But the illusion won't last long. In just ten days time darkness will already be as much as a whole hour longer than daylight, and then it's downhill all the way to Christmas. Let joy be unconfined.

<< click for Newer posts

click for Older Posts >>

click to return to the main page

...or read more in my monthly archives
Jan21  Feb21  Mar21  Apr21  May21  Jun21  Jul21  Aug21  Sep21  Oct21  Nov21
Jan20  Feb20  Mar20  Apr20  May20  Jun20  Jul20  Aug20  Sep20  Oct20  Nov20  Dec20
Jan19  Feb19  Mar19  Apr19  May19  Jun19  Jul19  Aug19  Sep19  Oct19  Nov19  Dec19
Jan18  Feb18  Mar18  Apr18  May18  Jun18  Jul18  Aug18  Sep18  Oct18  Nov18  Dec18
Jan17  Feb17  Mar17  Apr17  May17  Jun17  Jul17  Aug17  Sep17  Oct17  Nov17  Dec17
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

twenty blogs
ian visits
blue witch
the great wen
edith's streets
spitalfields life
in the aquarium
round the island
wanstead meteo
christopher fowler
bus and train user
ruth's coastal walk
the ladies who bus
round the rails we go
london reconnections
dirty modern scoundrel
from the murky depths

quick reference features
Things to do in Outer London
Things to do outside London
Inner London toilet map
The DG Tour of Britain

read the archive
Nov21  Oct21  Sep21
Aug21  Jul21  Jun21  May21
Apr21  Mar21  Feb21  Jan21
Dec20  Nov20  Oct20  Sep20
Aug20  Jul20  Jun20  May20
Apr20  Mar20  Feb20  Jan20
Dec19  Nov19  Oct19  Sep19
Aug19  Jul19  Jun19  May19
Apr19  Mar19  Feb19  Jan19
Dec18  Nov18  Oct18  Sep18
Aug18  Jul18  Jun18  May18
Apr18  Mar18  Feb18  Jan18
Dec17  Nov17  Oct17  Sep17
Aug17  Jul17  Jun17  May17
Apr17  Mar17  Feb17  Jan17
Dec16  Nov16  Oct16  Sep16
Aug16  Jul16  Jun16  May16
Apr16  Mar16  Feb16  Jan16
Dec15  Nov15  Oct15  Sep15
Aug15  Jul15  Jun15  May15
Apr15  Mar15  Feb15  Jan15
Dec14  Nov14  Oct14  Sep14
Aug14  Jul14  Jun14  May14
Apr14  Mar14  Feb14  Jan14
Dec13  Nov13  Oct13  Sep13
Aug13  Jul13  Jun13  May13
Apr13  Mar13  Feb13  Jan13
Dec12  Nov12  Oct12  Sep12
Aug12  Jul12  Jun12  May12
Apr12  Mar12  Feb12  Jan12
Dec11  Nov11  Oct11  Sep11
Aug11  Jul11  Jun11  May11
Apr11  Mar11  Feb11  Jan11
Dec10  Nov10  Oct10  Sep10
Aug10  Jul10  Jun10  May10
Apr10  Mar10  Feb10  Jan10
Dec09  Nov09  Oct09  Sep09
Aug09  Jul09  Jun09  May09
Apr09  Mar09  Feb09  Jan09
Dec08  Nov08  Oct08  Sep08
Aug08  Jul08  Jun08  May08
Apr08  Mar08  Feb08  Jan08
Dec07  Nov07  Oct07  Sep07
Aug07  Jul07  Jun07  May07
Apr07  Mar07  Feb07  Jan07
Dec06  Nov06  Oct06  Sep06
Aug06  Jul06  Jun06  May06
Apr06  Mar06  Feb06  Jan06
Dec05  Nov05  Oct05  Sep05
Aug05  Jul05  Jun05  May05
Apr05  Mar05  Feb05  Jan05
Dec04  Nov04  Oct04  Sep04
Aug04  Jul04  Jun04  May04
Apr04  Mar04  Feb04  Jan04
Dec03  Nov03  Oct03  Sep03
Aug03  Jul03  Jun03  May03
Apr03  Mar03  Feb03  Jan03
Dec02  Nov02  Oct02  Sep02
back to main page

the diamond geezer index
2020 2019 2018 2017 2016
2015 2014 2013 2012 2011
2010 2009 2008 2007 2006
2005 2004 2003 2002

my special London features
a-z of london museums
E3 - local history month
greenwich meridian (N)
greenwich meridian (S)
the real eastenders
london's lost rivers
olympic park 2007
great british roads
oranges & lemons
random boroughs
bow road station
high street 2012
river westbourne
trafalgar square
capital numbers
east london line
lea valley walk
olympics 2005
regent's canal
square routes
silver jubilee
unlost rivers
cube routes
Herbert Dip
capital ring
river fleet

ten of my favourite posts
the seven ages of blog
my new Z470xi mobile
five equations of blog
the dome of doom
chemical attraction
quality & risk
london 2102
single life
april fool

ten sets of lovely photos
my "most interesting" photos
london 2012 olympic zone
harris and the hebrides
betjeman's metro-land
marking the meridian
tracing the river fleet
london's lost rivers
inside the gherkin
seven sisters

just surfed in?
here's where to find...
diamond geezers
flash mob #1  #2  #3  #4
ben schott's miscellany
london underground
watch with mother
cigarette warnings
digital time delay
wheelie suitcases
war of the worlds
transit of venus
top of the pops
old buckenham
ladybird books
acorn antiques
digital watches
outer hebrides
olympics 2012
school dinners
pet shop boys
west wycombe
bletchley park
george orwell
big breakfast
clapton pond
san francisco
children's tv
east enders
trunk roads
little britain
credit cards
jury service
big brother
jubilee line
number 1s
titan arum
doctor who
blue peter
peter pan
feng shui
leap year
bbc three
vision on
ID cards