Why doesn't the latest Sunrise happen at the Winter Solstice ?

There's always some surprise when people discover that the "shortest day" of the year (the Winter Solstice) isn't the day with the latest Sunrise nor the day with the earliest Sunset.

Whitaker's Almanac is a good source of Sunrise and Sunset times. The latest copy I have access to is that for 1993, but the Earth goes round the Sun in the same sort of way now as it did in 1993...

Notes:

  1. Sunrise/Sunset times are only given to the nearest minute
  2. Winter Solstice was 20:26 GMT 21-Dec-1993
Greenwich Mean Time
DecemberLondon SunriseLondon SunsetLength of Day
57:4915:5308:04
67:5015:5208:02
77:5115:5208:01
87:5315:5207:59
97:5415:5207:58
107:5515:5107:56
117:5615:5107:55
127:5715:5107:54
137:5815:5107:53
147:5915:5107:52
158:0015:5107:51
168:0015:5207:52
178:0115:5207:51
188:0215:5207:50
198:0215:5307:51
208:0315:5307:50
218:0415:5307:49
228:0415:5407:50
238:0515:5407:49
248:0515:5507:50
258:0515:5607:51
268:0615:5607:50
278:0615:5707:51
288:0615:5807:52
298:0615:5907:53
308:0616:0007:54
318:0616:0107:55

So what's happening ? Why does the earliest Sunset happen about 8 days before the Solstice, and the latest Sunrise about 8 days after the Solstice ?

Let's see what happens when we take the Equation of Time into consideration. We have to add the Equation of Time to the Mean Time to get the Solar Time.
Greenwich Mean TimeGreenwich Solar Time
DecemberLondon SunriseLondon SunsetLength of DayEquation of Time (mm:ss)London SunriseLondon Sunset
57:4915:5308:0409:3207:58:3216:02:32
67:5015:5208:0209:0707:59:0716:01:07
77:5115:5208:0108:4207:59:4216:00:42
87:5315:5207:5908:1608:01:1616:00:16
97:5415:5207:5807:4908:01:4915:59:49
107:5515:5107:5607:2308:02:2315:58:23
117:5615:5107:5506:5508:02:5515:57:55
127:5715:5107:5406:2708:03:2715:57:27
137:5815:5107:5305:5908:03:5915:56:59
147:5915:5107:5205:3108:04:3115:56:31
158:0015:5107:5105:0208:05:0215:56:02
168:0015:5207:5204:3308:04:3315:56:33
178:0115:5207:5104:0408:05:0415:56:04
188:0215:5207:5003:3408:05:3415:55:34
198:0215:5307:5103:0508:05:0515:56:05
208:0315:5307:5002:3508:05:3515:55:35
218:0415:5307:4902:0508:06:0515:55:05
228:0415:5407:5001:3508:05:3515:55:35
238:0515:5407:4901:0608:06:0615:55:06
248:0515:5507:5000:3608:05:3615:55:36
258:0515:5607:5100:0608:05:0615:56:06
268:0615:5607:50-00:2408:05:3615:56:24
278:0615:5707:51-00:5308:05:0715:57:53
288:0615:5807:52-01:2308:04:3715:59:23
298:0615:5907:53-01:5208:04:0816:00:52
308:0616:0007:54-02:2108:03:3916:02:21
318:0616:0107:55-02:5008:03:1016:03:50

Ta-Da! It's the Equation of Time which is giving rise to the effect. If we consider Solar Time then the Solstice, the latest Sunrise, and the earliest Sunset happen together.

It would be more impressive (and more accurate) if I had Sunrise/Sunset figures which were accurate to the nearest second. You can treat this as a request...

For more info see http://www.rog.nmm.ac.uk/leaflets/special/sunrise.html

A similar effect occurs at the Summer Solstice, except in the summer we're not cold and stuck indoors thinking about why the days are so short...


Last updated: Monday, 03-Jan-2005 17:29:54 CET