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L O N G E S T   D A Y   -   S H O R T E S T   D A Y   -   are the same length. Their names exist because they are the days with the longest period of daylight and shortest period of daylight. The word solstice is a corruption of 2 latin words that mean sun and stand.

   The days when this occurs are known as the Summer Solstice, about June 21, and the Winter Solstice, about December 21. These dates/days do vary, and so does the time of whichever day.

   Very basically the Earth revolves round the Sun, also the Earth's axis is tilted relative to the Sun at 23.45. The relative tilt of the Earth's varies to the supposed axis of the Sun as we progress through the year.
   In what we call an Earth day the earth apparently rotates roughly once so we see sunrise in what we call the morning and sunset in what we call the evening.    There are 365 days in a year, so every year we have a new cycle - not that simple though because it is closer to 365¼ days per year which is why every 4th year we add the odd ¼ s together to give an extra day which is of course our leap year. If we didn't make this basic adjustment, every 700 or so years the winter would be in the summer, and the summer in the winter.
   Even that is not exact and every so often a leap second has to to be added to adjust that small but increasing inaccuracy.

   With the respective movements of Sun and Earth at these significant times during the year the northern and southern hemispheres are nearer the Sun, or the Sun is further north or south.
   In the northern hemisphere, when the Sun is furthest north in June at the time of the Summer Solstice the Sun is overhead at latitude 23 27' north, also known as the Tropic of Cancer.
   There is no convienient arrangement to make a precise adjustment to our year every year so as they roll by the actual time of the of the Solstice actually becomes slightly early until adjustment of the leap year moves it back. The consequence of this is that a summer solstice on June 22 will gradually change to June 21 and earlier, a winter solstice from December 22 to December 21 and earlier.
   Even after the leap-year adjustments take effect you begin to see over many years a change of solstice time which is demonstrated in the graph below.
   Leap year adjustments usually take place when the year, such as 1988, is divisible by 4. However the rule for leap years when a century occurs, such as 1900, is that it must be exactly divisible by 400 and 1900 is not, whereas 2000 is and therefore allows the solstice to slip further forward in the year. If there had been no leap year adjustment the date/time would be even earlier.

   An apparent anomaly in all of this is that the Sun does not set at its latest on the longest day or at its earliest on the shortest day. The reason is complex and revolves around the Sun's reduction or increase of the interval of the Sun's transit and setting, and also of the change of the equation of time. It all balances out.

   It should be noted that the times in the graph are Greenwich Mean Time and take no account of the annual change to the outdated British Summer Time, which should be made permanent as British Standard Time. Changing back and forth between GMT and BST does of course mean the last Sunday in March has 23 hours and the last Sunday in October has 25 hours.

   We have created a table below to show the Summer Solstice, the Winter Solstice, and both Vernal and Autumnal Equinoxes. The Equinox is in fact the point when the Sun passes over the Equator.
   Since there are so many variables to all of this, it should be stressed there is no absolute date/time for any of these events and different algorithms give slightly differing results, so we presume to say no table is spot-on, just close.

 
 
Year
 
Vernal equinox
 
Summer solstice
 
Autumnal equinox
 
Winter solstice
  
Date
Time
 
Date
Time
 
Date
Time
 
Date
Time
 
2000
 
Mar.20
07:26
 
Jun.21
01:37
 
Sep.22
17:12
 
Dec.21
13:25
2001
 
Mar.20
13:15
 
Jun.21
07:25
 
Sep.22
23:01
 
Dec.21
19:15
2002
 
Mar.20
19:04
 
Jun.21
13:13
 
Sep.23
04:49
 
Dec.22
01:04
2003
 
Mar.21
00:53
 
Jun.21
19:01
 
Sep.23
10:38
 
Dec.22
06:54
2004
 
Mar.20
06:42
 
Jun.21
00:49
 
Sep.22
16:26
 
Dec.21
12:43
2005
 
Mar.20
12:31
 
Jun.21
06:37
 
Sep.22
22:15
 
Dec.21
18:33
2006
 
Mar.20
18:20
 
Jun.21
12:25
 
Sep.23
04:03
 
Dec.22
00:22
2007
 
Mar.21
00:09
 
Jun.21
18:13
 
Sep.23
09:52
 
Dec.22
06:12
2008
 
Mar.20
05:58
 
Jun.21
00:00
 
Sep.22
15:40
 
Dec.21
12:02
2009
 
Mar.20
11:47
 
Jun.21
05:48
 
Sep.22
21:29
 
Dec.21
17:51
2010
 
Mar.20
17:36
 
Jun.21
11:36
 
Sep.23
03:17
 
Dec.21
23:41
2011
 
Mar.20
23:25
 
Jun.21
17:24
 
Sep.23
09:06
 
Dec.22
05:30
2012
 
Mar.20
05:14
 
Jun.20
23:12
 
Sep.22
14:54
 
Dec.21
11:20
2013
 
Mar.20
11:03
 
Jun.21
05:00
 
Sep.22
20:43
 
Dec.21
17:09
2014
 
Mar.20
16:52
 
Jun.21
10:48
 
Sep.23
02:31
 
Dec.21
22:59
2015
 
Mar.20
22:41
 
Jun.21
16:36
 
Sep.23
08:20
 
Dec.22
04:48
2016
 
Mar.20
04:30
 
Jun.20
22:24
 
Sep.22
14:08
 
Dec.21
10:38
2017
 
Mar.20
10:19
 
Jun.21
04:12
 
Sep.22
19:57
 
Dec.21
16:27
2018
 
Mar.20
16:08
 
Jun.21
10:00
 
Sep.23
01:45
 
Dec.21
22:17
2019
 
Mar.20
21:57
 
Jun.21
15:48
 
Sep.23
07:34
 
Dec.22
04:07
2020
 
Mar.20
03:46
 
Jun.20
21:36
 
Sep.22
13:22
 
Dec.21
09:56
 
 
 
 
 

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