The Equation of Time
This is a table of the amount by which the Sun is fast or slow each day compared with the average time that we use for our everyday life – and which is why, for example, Greenwich Mean Time is the mean, or average, of the variations in solar time.

The reason this is necessary is that the Earth's orbit around the Sun is not a perfect circle but is an ellipse. When the Earth is comparatively close to the Sun (as it is in January), it moves faster in its orbit than when it is farther from the Sun, as in June.

As a result, when the Earth is moving faster than average in its orbit, the Earth doesn't rotate on its axis quite enough during one day to bring the Sun due south at 12 noon by our watches, and it is slow.

Here is a table giving the approximate dates when the Sun is fast (–) or slow (+) compared with your watch by a whole number of minutes:

 Jan 1     3 Jan 3     4 Jan 5     5 Jan 8     6 Jan 10   7 Jan 12   8 Jan 15   9 Jan 18 10 Jan 2   11 Jan 2   12 Jan 29   13 Feb 5    14 Feb 26  13 Mar 4    12 Mar 8    11 Mar 2    10 Mar 16    9 Mar 19    8 Mar 22    7 Mar 26    6 Mar 29   5 Apr 1      4 Apr 5      3 Apr 8      2 Apr 12    1 Apr 16    0 Apr 21  –1 Apr 26  –2 May 3  – 3 Jun 3   – 2 Jun 9      –1 Jun 14    –0 Jun 19      1 Jun 24      2 Jun 29      3 Jul 4         4 Jul 10       5 Jul 18       6 Aug 12     5 Aug 17     4 Aug 22     3 Aug 26     2 Aug 29     1 Sep 2       0 Sep 5     –1 Sep 8     –2 Sep 11   –3 Sep 14   –4 Sep 17   –5 Sep 20   –6 Sep 22  –7 Sep 25  –8 Sep 28  –9 Oct 1    –10 Oct 4    –11 Oct 8    –12 Oct 11  –13 Oct 15  –14 Oct 20  –15 Oct 27  –16 Nov 17  –15 Nov 22  –14 Nov 25  –13 Nov 28  –12 Dec 1    –11 Dec 4    –10 Dec 6      –9 Dec 8      –8 Dec 11    –7 Dec 13    –6 Dec 15    –5 Dec 17    –4 Dec 19    –3 Dec 21    –2 Dec 23    –1 Dec 25      0 Dec 28      1 Dec 30      2

How does this affect you? If you are trying to find your north-south line by day, make the correction to find out when the Sun is exactly on your meridian. For example, on 2 March the Sun is due south at 12.10 pm. Or if you have a well-made sundial, add or subtract the figures shown to get the local civil time. If the sundial reads 3.45 on 17 November, the time by your watch should be 3.30 pm.

Actually, there are other factors affecting sundial time, of which the main one is shoddy manufacture, which is the problem with most of the sundials you buy in garden centres these days... but don't get me started, that's another story. With a well-made sundial, the other correction you must make is for your distance from the meridian of the time zone you are in. Even in a small country such as the UK this can be considerable. Remember that every 15º of longitude makes an hour of difference in the solar time, or 4 minutes per degree. So in Cornwall, west Wales, western Scotland or Northern Ireland you are about 5º west of the Greenwich meridian and the solar time is 20 minutes later than GMT. On 5 February in Belfast, therefore, a sundial will not tell you that it is noon until 12.35 on your watch, and in early August it will be 1.28 pm when the sundial reads noon, because of British Summer Time.

Once you have made these corrections, a properly made sundial should give you the time just as accurately as most digital watches, which can easily drift off by half a minute or more unless they are regularly checked.

For a full listing of the Equation of Time for every day in the year go to
http://freepages.pavilion.net/users/aghelyar/sundat.htm (separate site).
What's in the sky tonight?
Below is a listing of the positions of the planets for this year. They are calculated for an observer at the centre of the Earth, so they apply equally to everybody -- the difference between locations does not matter at this level of accuracy. But if you just want to know what the sky will look like tonight, where you are, try the website www.heavens-above.com. Once you have registered (free) with your own location you can get a whole-sky map (under Astronomy) and get your own predictions for bright satellites such as the International Space Station and the Iridium satellites which flare in brightness.

Positions calculated using Chris Marriott's SkyMap

Positions of the planets for 2008
Mercury  – positions every 5 days during
western and eastern elongations.  El = elongation from Sun in degrees.

 Date Geo R.A. Geo Dec Mag El ° Eastern elongation – evening sky after sunset 11 Jan 2008 20h 29m 20.15s -20° 57' 50.3" -0.9 14.5 16 Jan 2008 21h 00m 23.42s -18° 26' 09.0" -0.8 17.1 21 Jan 2008 21h 25m 49.01s -15° 37' 31.3" -0.6 18.6 26 Jan 2008 21h 40m 52.40s -13° 04' 35.0" -0.1 17.8 Western elongation – morning sky before sunrise 15 Feb 2008 20h 43m 51.65s -14° 56' 23.1" 1.6 16.6 20 Feb 2008 20h 40m 53.65s -16° 09' 35.4" 0.7 22.5 25 Feb 2008 20h 49m 01.03s -16° 43' 25.4" 0.3 25.7 01 Mar 2008 21h 04m 53.41s -16° 37' 40.0" 0.2 27.0 06 Mar 2008 21h 25m 50.98s -15° 54' 22.5" 0.1 27.0 11 Mar 2008 21h 50m 08.57s -14° 35' 40.9" 0.0 26.1 16 Mar 2008 22h 16m 42.38s -12° 43' 27.3" -0.0 24.4 21 Mar 2008 22h 44m 57.09s -10° 19' 19.6" -0.2 22.1 26 Mar 2008 23h 14m 37.52s -07° 24' 46.6" -0.3 19.2 31 Mar 2008 23h 45m 43.47s -04° 01' 24.7" -0.6 15.7 Eastern elongation – evening sky after sunset 30 Apr 2008 03h 28m 27.39s +20° 38' 11.8" -1.0 15.0 05 May 2008 04h 04m 25.99s +23° 05' 35.2" -0.5 18.8 10 May 2008 04h 35m 04.08s +24° 31' 52.2" -0.0 21.2 15 May 2008 04h 58m 55.95s +25° 03' 43.3" 0.6 21.8 20 May 2008 05h 14m 57.97s +24° 50' 09.4" 1.2 20.5 25 May 2008 05h 22m 27.74s +23° 59' 48.3" 2.1 17.3 Western elongation – morning sky before sunrise 19 Jun 2008 04h 48m 35.09s +17° 58' 46.8" 2.4 15.7 24 Jun 2008 04h 51m 32.95s +18° 15' 29.1" 1.5 19.6 29 Jun 2008 05h 02m 30.62s +19° 08' 30.4" 0.8 21.5 04 Jul 2008 05h 21m 26.86s +20° 23' 29.0" 0.2 21.6 09 Jul 2008 05h 48m 12.15s +21° 42' 12.1" -0.3 20.0 14 Jul 2008 06h 22m 27.02s +22° 42' 52.9" -0.8 16.8 Eastern elongation – evening sky after sunset 18 Aug 2008 10h 59m 04.82s +07° 21' 21.2" -0.4 17.7 23 Aug 2008 11h 27m 31.75s +03° 41' 43.0" -0.2 20.9 28 Aug 2008 11h 53m 27.39s +00° 08' 59.9" -0.0 23.4 02 Sep 2008 12h 17m 00.55s -03° 11' 40.1" 0.0 25.3 07 Sep 2008 12h 38m 02.99s -06° 14' 53.6" 0.1 26.5 12 Sep 2008 12h 56m 02.90s -08° 53' 52.1" 0.2 26.9 17 Sep 2008 13h 09m 54.65s -10° 58' 38.5" 0.3 26.0 22 Sep 2008 13h 17m 47.28s -12° 13' 33.0" 0.6 23.5 27 Sep 2008 13h 17m 08.76s -12° 14' 33.5" 1.3 18.5 Western elongation – morning sky before sunrise 17 Oct 2008 12h 29m 58.48s -02° 02' 41.8" 0.4 16.3 22 Oct 2008 12h 42m 27.30s -02° 30' 59.0" -0.5 18.3 27 Oct 2008 13h 04m 52.66s -04° 37' 32.5" -0.8 17.3 Eastern elongation – evening sky after sunset 26 Dec 2008 19h 32m 18.96s -23° 52' 30.8" -0.7 16.6 31 Dec 2008 20h 02m 30.77s -22° 11' 34.4" -0.7 18.6
Venus – positions every 10 days El = elongation from Sun in degrees.
 Date Geo R.A. Geo Dec Mag El ° 01 Jan 2008 15h 59m 19.18s -18° 28' 11.1" -4.1 38.5 11 Jan 2008 16h 50m 02.71s -20° 45' 22.9" -4.0 36.5 21 Jan 2008 17h 42m 19.27s -22° 06' 28.2" -4.0 34.4 31 Jan 2008 18h 35m 27.56s -22° 24' 06.7" -4.0 32.3 10 Feb 2008 19h 28m 34.65s -21° 35' 26.4" -4.0 30.1 20 Feb 2008 20h 20m 47.61s -19° 42' 48.6" -3.9 27.9 01 Mar 2008 21h 11m 31.27s -16° 52' 51.5" -3.9 25.6 11 Mar 2008 22h 00m 31.52s -13° 15' 13.1" -3.9 23.2 21 Mar 2008 22h 47m 53.52s -09° 01' 21.0" -3.9 20.8 31 Mar 2008 23h 34m 00.91s -04° 22' 59.6" -3.9 18.4 10 Apr 2008 00h 19m 27.50s +00° 28' 04.3" -3.9 15.9 Too close to Sun for observation 08 Aug 2008 10h 19m 02.82s +12° 03' 28.7" -3.9 16.5 18 Aug 2008 11h 05m 08.22s +07° 22' 26.6" -3.9 19.2 28 Aug 2008 11h 50m 07.40s +02° 21' 41.0" -3.9 21.8 07 Sep 2008 12h 34m 40.07s -02° 46' 57.0" -3.9 24.3 17 Sep 2008 13h 19m 26.91s -07° 51' 25.4" -3.9 26.8 27 Sep 2008 14h 05m 09.01s -12° 39' 46.1" -3.9 29.3 07 Oct 2008 14h 52m 21.62s -16° 59' 34.4" -3.9 31.6 17 Oct 2008 15h 41m 25.02s -20° 38' 01.4" -4.0 33.9 27 Oct 2008 16h 32m 21.44s -23° 22' 56.5" -4.0 36.1 06 Nov 2008 17h 24m 46.98s -25° 03' 46.0" -4.0 38.1 16 Nov 2008 18h 17m 48.02s -25° 33' 26.9" -4.1 40.1 26 Nov 2008 19h 10m 16.72s -24° 50' 04.2" -4.1 41.9 06 Dec 2008 20h 01m 05.57s -22° 57' 13.0" -4.2 43.5 16 Dec 2008 20h 49m 18.73s -20° 03' 33.8" -4.2 44.9 26 Dec 2008 21h 34m 25.22s -16° 20' 50.9" -4.3 46.1

Mars – positions every 10 days

 Date Geo R.A. Geo Dec Mag 01 Jan 2008 05h 59m 27.42s +26° 56' 27.6" -1.5 11 Jan 2008 05h 45m 14.20s +26° 58' 11.4" -1.2 21 Jan 2008 05h 36m 22.34s +26° 50' 56.8" -0.9 31 Jan 2008 05h 33m 31.28s +26° 41' 11.5" -0.6 10 Feb 2008 05h 36m 17.53s +26° 32' 17.1" -0.3 20 Feb 2008 05h 43m 53.41s +26° 24' 22.9" -0.1 01 Mar 2008 05h 55m 20.88s +26° 15' 43.5" 0.2 11 Mar 2008 06h 09m 51.96s +26° 03' 50.4" 0.4 21 Mar 2008 06h 26m 46.77s +25° 46' 08.7" 0.6 31 Mar 2008 06h 45m 29.37s +25° 20' 28.1" 0.8 10 Apr 2008 07h 05m 34.13s +24° 45' 02.9" 0.9 20 Apr 2008 07h 26m 39.17s +23° 58' 36.1" 1.1 30 Apr 2008 07h 48m 25.24s +23° 00' 25.7" 1.2 10 May 2008 08h 10m 39.62s +21° 50' 06.3" 1.3 20 May 2008 08h 33m 10.88s +20° 27' 40.5" 1.4 30 May 2008 08h 55m 50.10s +18° 53' 32.0" 1.5 09 Jun 2008 09h 18m 32.92s +17° 08' 08.4" 1.5 19 Jun 2008 09h 41m 15.14s +15° 12' 20.0" 1.6 29 Jun 2008 10h 03m 55.01s +13° 07' 03.9" 1.6 09 Jul 2008 10h 26m 33.87s +10° 53' 14.3" 1.7 19 Jul 2008 10h 49m 12.51s +08° 32' 04.6" 1.7 29 Jul 2008 11h 11m 53.80s +06° 04' 46.5" 1.7 08 Aug 2008 11h 34m 42.54s +03° 32' 30.0" 1.7 18 Aug 2008 11h 57m 42.67s +00° 56' 43.2" 1.7 28 Aug 2008 12h 20m 59.83s -01° 41' 09.8" 1.7 07 Sep 2008 12h 44m 40.67s -04° 19' 39.2" 1.7 17 Sep 2008 13h 08m 50.78s -06° 56' 57.7" 1.7 27 Sep 2008 13h 33m 37.16s -09° 31' 18.7" 1.6 07 Oct 2008 13h 59m 06.74s -12° 00' 42.3" 1.6 Too close to Sun for observation

Jupiter – positions every 10 days

 Date Geo R.A. Geo Dec Mag 01 Jan 2008 18h 13m 07.72s -23° 14' 12.7" -1.8 11 Jan 2008 18h 23m 01.96s -23° 10' 48.9" -1.8 21 Jan 2008 18h 32m 45.01s -23° 05' 12.1" -1.8 31 Jan 2008 18h 42m 10.68s -22° 57' 37.7" -1.9 10 Feb 2008 18h 51m 12.83s -22° 48' 24.8" -1.9 20 Feb 2008 18h 59m 44.47s -22° 37' 59.5" -1.9 01 Mar 2008 19h 07m 39.30s -22° 26' 51.5" -2.0 11 Mar 2008 19h 14m 50.70s -22° 15' 34.0" -2.0 21 Mar 2008 19h 21m 11.59s -22° 04' 44.0" -2.1 31 Mar 2008 19h 26m 35.67s -21° 54' 57.6" -2.1 10 Apr 2008 19h 30m 56.12s -21° 46' 52.1" -2.2 20 Apr 2008 19h 34m 06.72s -21° 41' 02.5" -2.3 30 Apr 2008 19h 36m 02.71s -21° 37' 54.5" -2.4 10 May 2008 19h 36m 39.84s -21° 37' 48.6" -2.4 20 May 2008 19h 35m 56.79s -21° 40' 51.6" -2.5 30 May 2008 19h 33m 55.76s -21° 46' 53.0" -2.6 09 Jun 2008 19h 30m 42.15s -21° 55' 29.2" -2.6 19 Jun 2008 19h 26m 27.31s -22° 05' 58.4" -2.7 29 Jun 2008 19h 21m 27.16s -22° 17' 29.3" -2.7 09 Jul 2008 19h 16m 01.52s -22° 29' 08.3" -2.7 19 Jul 2008 19h 10m 33.95s -22° 40' 03.0" -2.7 29 Jul 2008 19h 05m 27.33s -22° 49' 35.9" -2.7 08 Aug 2008 19h 01m 03.11s -22° 57' 23.2" -2.6 18 Aug 2008 18h 57m 39.36s -23° 03' 12.1" -2.6 28 Aug 2008 18h 55m 28.02s -23° 07' 02.2" -2.5 07 Sep 2008 18h 54m 36.86s -23° 08' 55.9" -2.5 17 Sep 2008 18h 55m 08.77s -23° 08' 54.9" -2.4 27 Sep 2008 18h 57m 02.22s -23° 06' 59.6" -2.3 07 Oct 2008 19h 00m 13.83s -23° 03' 04.7" -2.3 17 Oct 2008 19h 04m 37.67s -22° 57' 02.9" -2.2 27 Oct 2008 19h 10m 06.68s -22° 48' 45.1" -2.1 06 Nov 2008 19h 16m 33.83s -22° 37' 58.5" -2.1 16 Nov 2008 19h 23m 50.98s -22° 24' 34.2" -2.0 26 Nov 2008 19h 31m 50.52s -22° 08' 24.3" -2.0 06 Dec 2008 19h 40m 25.29s -21° 49' 22.2" -2.0 16 Dec 2008 19h 49m 27.57s -21° 27' 27.7" -2.0 26 Dec 2008 19h 58m 50.62s -21° 02' 43.6" -1.9

Saturn – positions every 10 days

 Date Geo R.A. Geo Dec Mag 01 Jan 2008 10h 42m 47.92s +09° 58' 15.1" 0.6 11 Jan 2008 10h 41m 40.45s +10° 07' 31.2" 0.5 21 Jan 2008 10h 39m 55.82s +10° 20' 15.3" 0.4 31 Jan 2008 10h 37m 39.59s +10° 35' 47.1" 0.4 10 Feb 2008 10h 34m 58.81s +10° 53' 18.0" 0.3 20 Feb 2008 10h 32m 02.57s +11° 11' 48.2" 0.2 01 Mar 2008 10h 29m 00.78s +11° 30' 16.3" 0.2 11 Mar 2008 10h 26m 03.59s +11° 47' 43.2" 0.3 21 Mar 2008 10h 23m 21.15s +12° 03' 11.8" 0.3 31 Mar 2008 10h 21m 01.98s +12° 15' 58.7" 0.3 10 Apr 2008 10h 19m 13.13s +12° 25' 30.1" 0.4 20 Apr 2008 10h 18m 00.08s +12° 31' 21.8" 0.5 30 Apr 2008 10h 17m 25.78s +12° 33' 24.2" 0.5 10 May 2008 10h 17m 31.73s +12° 31' 33.8" 0.6 20 May 2008 10h 18m 17.82s +12° 25' 55.4" 0.6 30 May 2008 10h 19m 42.44s +12° 16' 40.2" 0.7 09 Jun 2008 10h 21m 43.41s +12° 04' 00.5" 0.7 19 Jun 2008 10h 24m 17.53s +11° 48' 14.1" 0.7 29 Jun 2008 10h 27m 21.22s +11° 29' 40.0" 0.8 09 Jul 2008 10h 30m 51.03s +11° 08' 35.8" 0.8 19 Jul 2008 10h 34m 42.83s +10° 45' 23.6" 0.8 29 Jul 2008 10h 38m 52.67s +10° 20' 24.6" 0.8 08 Aug 2008 10h 43m 16.85s +09° 53' 59.3" 0.8 Too close to Sun for observation 27 Sep 2008 11h 06m 33.73s +07° 34' 14.8" 0.9 07 Oct 2008 11h 11m 00.84s +07° 07' 42.2" 1.0 17 Oct 2008 11h 15m 14.32s +06° 42' 41.8" 1.0 27 Oct 2008 11h 19m 10.28s +06° 19' 39.9" 1.0 06 Nov 2008 11h 22m 44.34s +05° 59' 06.3" 1.1 16 Nov 2008 11h 25m 52.30s +05° 41' 28.9" 1.1 26 Nov 2008 11h 28m 30.38s +05° 27' 12.5" 1.1 06 Dec 2008 11h 30m 34.68s +05° 16' 41.8" 1.0 16 Dec 2008 11h 32m 02.12s +05° 10' 15.0" 1.0 26 Dec 2008 11h 32m 50.34s +05° 08' 04.5" 1.0