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

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

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 ?

• The Earth rotates about its axis once a day.
• Each day, the Earth moves along its orbit path (as a consequence, to put the Sun "in the same place" each day the Earth has to do a little more than exactly one rotation).
• If the Earth was in a circular orbit, things would be easier, but as the Earth is in an elliptical orbit the speed at which the Earth moves along its orbit path isn't constant. As a consequence, "Solar Days" aren't the same length throughout the year.
• If you are using the Sun for time and date measurement, you'd really like to have days of the same length. This is where "mean time" comes in (as in "Greenwich Mean Time").
• Mean time is derived by applying a correction to Solar Time in order to make each day the same length. The correction is called "the Equation of Time". The Equation of Time, if plotted over a period of a year, is zero 4 times a year, and has maximum offsets of about +/- 15 minutes.
• The values of the Equation of Time change quite rapidly around the soltices.

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 Time				Greenwich Solar Time
December	London Sunrise	London Sunset	Length of Day	Equation of Time (mm:ss)	London Sunrise	London Sunset
5	7:49	15:53	08:04	09:32	07:58:32	16:02:32
6	7:50	15:52	08:02	09:07	07:59:07	16:01:07
7	7:51	15:52	08:01	08:42	07:59:42	16:00:42
8	7:53	15:52	07:59	08:16	08:01:16	16:00:16
9	7:54	15:52	07:58	07:49	08:01:49	15:59:49
10	7:55	15:51	07:56	07:23	08:02:23	15:58:23
11	7:56	15:51	07:55	06:55	08:02:55	15:57:55
12	7:57	15:51	07:54	06:27	08:03:27	15:57:27
13	7:58	15:51	07:53	05:59	08:03:59	15:56:59
14	7:59	15:51	07:52	05:31	08:04:31	15:56:31
15	8:00	15:51	07:51	05:02	08:05:02	15:56:02
16	8:00	15:52	07:52	04:33	08:04:33	15:56:33
17	8:01	15:52	07:51	04:04	08:05:04	15:56:04
18	8:02	15:52	07:50	03:34	08:05:34	15:55:34
19	8:02	15:53	07:51	03:05	08:05:05	15:56:05
20	8:03	15:53	07:50	02:35	08:05:35	15:55:35
21	8:04	15:53	07:49	02:05	08:06:05	15:55:05
22	8:04	15:54	07:50	01:35	08:05:35	15:55:35
23	8:05	15:54	07:49	01:06	08:06:06	15:55:06
24	8:05	15:55	07:50	00:36	08:05:36	15:55:36
25	8:05	15:56	07:51	00:06	08:05:06	15:56:06
26	8:06	15:56	07:50	-00:24	08:05:36	15:56:24
27	8:06	15:57	07:51	-00:53	08:05:07	15:57:53
28	8:06	15:58	07:52	-01:23	08:04:37	15:59:23
29	8:06	15:59	07:53	-01:52	08:04:08	16:00:52
30	8:06	16:00	07:54	-02:21	08:03:39	16:02:21
31	8:06	16:01	07:55	-02:50	08:03:10	16:03:50

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...