Sundial Time Corrections

Corrections need to be made in order to obtain Mean time from a sundial. 

These are:

  • Correction for the Equation of Time
  • Correction for Longitude
  • Correction for Summer Time

These are considered in turn below:

Why not get the Estimator? Want a little card to carry with you to help you check a sundial's displayed time at ANY Longitude or Latitude in the British Isles?  Hurry if you want one of the few remaining printed versions...

Click the picture for more details!

Click to download the Estimator

The equation of time describes the discrepancy between two kinds of solar time. These are Apparent Solar Time, which directly tracks the motion of the sun, and Mean Solar Time, which tracks a fictitious "mean" sun with noons 24 hours apart. Apparent (or true) solar time is that indicated by a sundial. Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time average to zero. Why not get The Recorder's Reference? It provides anyone interested in dialling with a way to estimate by how much a non-direct-south vertical dial declines and also finds the design latitude for any horizontal dial. 

Click the picture for more details!

I - Equation of Time Correction - 2021 This page P.Powers 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 All Rights Reserved

The figures in the tables below show more accurate figures in mins and secs by which a sundial should be corrected during the whole of 2020.  A link is also included to a printable page of the same EoT figures. The figures in the tables represent the Equation of Time at 12Hrs (Noon) GMT on the date stated and are here calculated using the method after Smart (1956) and Meeus which is accurate at worst to about 4 secs - frequently to one second..  Interpolation will of course, be necessary if an EoT is needed for times other than Noon. Note that in non-leap-years there is still an entry in the tables for Feb 29th but its value is then the same as that for Mar 1st. NB Don't forget the two other corrections for longitude and daylight saving time that are needed to convert solar time to clock time...

 Negative values in the tables below indicate Dial Slow. Declination & EoTThibaud Taudin Chabot has prepared a two sheet print out of average values of the sun's declination and the equation of time at 12:00 GMT for the years 2000 - 2099. This can be found and printed off by clicking the image here.
2021 EoT Daily Entries One Page Printable version of 2021 EoT
Jan 01 -3m 41s May 01 2m 56s Sep 01 0m 3s
Jan 02 -4m 9s May 02 3m 2s Sep 02 0m 23s
Jan 03 -4m 37s May 03 3m 8s Sep 03 0m 42s
Jan 04 -5m 3s May 04 3m 14s Sep 04 1m 2s
Jan 05 -5m 30s May 05 3m 19s Sep 05 1m 22s
Jan 06 -5m 57s May 06 3m 24s Sep 06 1m 42s
Jan 07 -6m 23s May 07 3m 27s Sep 07 2m 3s
Jan 08 -6m 48s May 08 3m 31s Sep 08 2m 23s
Jan 09 -7m 13s May 09 3m 34s Sep 09 2m 44s
Jan 10 -7m 37s May 10 3m 36s Sep 10 3m 5s
Jan 11 -8m 1s May 11 3m 37s Sep 11 3m 26s
Jan 12 -8m 24s May 12 3m 38s Sep 12 3m 47s
Jan 13 -8m 47s May 13 3m 39s Sep 13 4m 8s
Jan 14 -9m 8s May 14 3m 39s Sep 14 4m 30s
Jan 15 -9m 30s May 15 3m 38s Sep 15 4m 51s
Jan 16 -9m 50s May 16 3m 37s Sep 16 5m 13s
Jan 17 -10m 10s May 17 3m 35s Sep 17 5m 34s
Jan 18 -10m 29s May 18 3m 33s Sep 18 5m 55s
Jan 19 -10m 48s May 19 3m 30s Sep 19 6m 17s
Jan 20 -11m 5s May 20 3m 26s Sep 20 6m 38s
Jan 21 -11m 23s May 21 3m 22s Sep 21 6m 59s
Jan 22 -11m 39s May 22 3m 18s Sep 22 7m 21s
Jan 23 -11m 54s May 23 3m 13s Sep 23 7m 42s
Jan 24 -12m 9s May 24 3m 7s Sep 24 8m 3s
Jan 25 -12m 23s May 25 3m 1s Sep 25 8m 23s
Jan 26 -12m 36s May 26 2m 55s Sep 26 8m 44s
Jan 27 -12m 48s May 27 2m 48s Sep 27 9m 5s
Jan 28 -12m 59s May 28 2m 40s Sep 28 9m 25s
Jan 29 -13m 10s May 29 2m 33s Sep 29 9m 45s
Jan 30 -13m 20s May 30 2m 24s Sep 30 10m 5s
Jan 31 -13m 29s May 31 2m 16s Oct 01 10m 24s
Feb 01 -13m 37s Jun 01 2m 6s Oct 02 10m 43s
Feb 02 -13m 44s Jun 02 1m 57s Oct 03 11m 2s
Feb 03 -13m 51s Jun 03 1m 47s Oct 04 11m 21s
Feb 04 -13m 57s Jun 04 1m 37s Oct 05 11m 39s
Feb 05 -14m 2s Jun 05 1m 26s Oct 06 11m 57s
Feb 06 -14m 6s Jun 06 1m 15s Oct 07 12m 15s
Feb 07 -14m 9s Jun 07 1m 4s Oct 08 12m 32s
Feb 08 -14m 11s Jun 08 0m 53s Oct 09 12m 48s
Feb 09 -14m 13s Jun 09 0m 41s Oct 10 13m 4s
Feb 10 -14m 14s Jun 10 0m 29s Oct 11 13m 20s
Feb 11 -14m 14s Jun 11 0m 17s Oct 12 13m 35s
Feb 12 -14m 13s Jun 12 0m 4s Oct 13 13m 50s
Feb 13 -14m 12s Jun 13 -0m 8s Oct 14 14m 4s
Feb 14 -14m 10s Jun 14 -0m 21s Oct 15 14m 18s
Feb 15 -14m 7s Jun 15 -0m 34s Oct 16 14m 31s
Feb 16 -14m 3s Jun 16 -0m 47s Oct 17 14m 43s
Feb 17 -13m 59s Jun 17 -0m 60s Oct 18 14m 55s
Feb 18 -13m 54s Jun 18 -1m 13s Oct 19 15m 6s
Feb 19 -13m 48s Jun 19 -1m 26s Oct 20 15m 17s
Feb 20 -13m 42s Jun 20 -1m 39s Oct 21 15m 27s
Feb 21 -13m 35s Jun 21 -1m 52s Oct 22 15m 36s
Feb 22 -13m 27s Jun 22 -2m 5s Oct 23 15m 45s
Feb 23 -13m 19s Jun 23 -2m 18s Oct 24 15m 53s
Feb 24 -13m 10s Jun 24 -2m 31s Oct 25 15m 60s
Feb 25 -13m 0s Jun 25 -2m 43s Oct 26 16m 6s
Feb 26 -12m 50s Jun 26 -2m 56s Oct 27 16m 12s
Feb 27 -12m 39s Jun 27 -3m 8s Oct 28 16m 17s
Feb 28 -12m 28s Jun 28 -3m 21s Oct 29 16m 21s
Feb 29 -12m 16s Jun 29 -3m 33s Oct 30 16m 24s
Mar 01 -12m 16s Jun 30 -3m 45s Oct 31 16m 27s
Mar 02 -12m 4s Jul 01 -3m 56s Nov 01 16m 28s
Mar 03 -11m 52s Jul 02 -4m 8s Nov 02 16m 29s
Mar 04 -11m 38s Jul 03 -4m 19s Nov 03 16m 29s
Mar 05 -11m 25s Jul 04 -4m 29s Nov 04 16m 29s
Mar 06 -11m 11s Jul 05 -4m 40s Nov 05 16m 27s
Mar 07 -10m 56s Jul 06 -4m 50s Nov 06 16m 25s
Mar 08 -10m 42s Jul 07 -4m 60s Nov 07 16m 21s
Mar 09 -10m 27s Jul 08 -5m 9s Nov 08 16m 17s
Mar 10 -10m 11s Jul 09 -5m 18s Nov 09 16m 12s
Mar 11 -9m 55s Jul 10 -5m 26s Nov 10 16m 6s
Mar 12 -9m 39s Jul 11 -5m 34s Nov 11 15m 60s
Mar 13 -9m 23s Jul 12 -5m 42s Nov 12 15m 52s
Mar 14 -9m 6s Jul 13 -5m 49s Nov 13 15m 44s
Mar 15 -8m 49s Jul 14 -5m 56s Nov 14 15m 34s
Mar 16 -8m 32s Jul 15 -6m 2s Nov 15 15m 24s
Mar 17 -8m 15s Jul 16 -6m 7s Nov 16 15m 13s
Mar 18 -7m 58s Jul 17 -6m 12s Nov 17 15m 1s
Mar 19 -7m 40s Jul 18 -6m 17s Nov 18 14m 49s
Mar 20 -7m 22s Jul 19 -6m 21s Nov 19 14m 35s
Mar 21 -7m 4s Jul 20 -6m 25s Nov 20 14m 21s
Mar 22 -6m 46s Jul 21 -6m 27s Nov 21 14m 6s
Mar 23 -6m 28s Jul 22 -6m 30s Nov 22 13m 50s
Mar 24 -6m 10s Jul 23 -6m 31s Nov 23 13m 33s
Mar 25 -5m 52s Jul 24 -6m 33s Nov 24 13m 16s
Mar 26 -5m 34s Jul 25 -6m 33s Nov 25 12m 57s
Mar 27 -5m 16s Jul 26 -6m 33s Nov 26 12m 38s
Mar 28 -4m 58s Jul 27 -6m 33s Nov 27 12m 19s
Mar 29 -4m 40s Jul 28 -6m 31s Nov 28 11m 58s
Mar 30 -4m 22s Jul 29 -6m 30s Nov 29 11m 37s
Mar 31 -4m 4s Jul 30 -6m 27s Nov 30 11m 15s
Apr 01 -3m 46s Jul 31 -6m 24s Dec 01 10m 53s
Apr 02 -3m 28s Aug 01 -6m 20s Dec 02 10m 30s
Apr 03 -3m 11s Aug 02 -6m 16s Dec 03 10m 6s
Apr 04 -2m 54s Aug 03 -6m 11s Dec 04 9m 42s
Apr 05 -2m 36s Aug 04 -6m 6s Dec 05 9m 17s
Apr 06 -2m 19s Aug 05 -5m 60s Dec 06 8m 52s
Apr 07 -2m 3s Aug 06 -5m 53s Dec 07 8m 26s
Apr 08 -1m 46s Aug 07 -5m 46s Dec 08 7m 60s
Apr 09 -1m 30s Aug 08 -5m 38s Dec 09 7m 33s
Apr 10 -1m 14s Aug 09 -5m 29s Dec 10 7m 6s
Apr 11 -0m 58s Aug 10 -5m 20s Dec 11 6m 38s
Apr 12 -0m 43s Aug 11 -5m 11s Dec 12 6m 10s
Apr 13 -0m 27s Aug 12 -5m 1s Dec 13 5m 42s
Apr 14 -0m 13s Aug 13 -4m 50s Dec 14 5m 13s
Apr 15 0m 2s Aug 14 -4m 39s Dec 15 4m 45s
Apr 16 0m 16s Aug 15 -4m 27s Dec 16 4m 16s
Apr 17 0m 30s Aug 16 -4m 15s Dec 17 3m 46s
Apr 18 0m 43s Aug 17 -4m 2s Dec 18 3m 17s
Apr 19 0m 56s Aug 18 -3m 49s Dec 19 2m 47s
Apr 20 1m 9s Aug 19 -3m 35s Dec 20 2m 18s
Apr 21 1m 21s Aug 20 -3m 20s Dec 21 1m 48s
Apr 22 1m 32s Aug 21 -3m 6s Dec 22 1m 18s
Apr 23 1m 44s Aug 22 -2m 51s Dec 23 0m 49s
Apr 24 1m 54s Aug 23 -2m 35s Dec 24 0m 19s
Apr 25 2m 5s Aug 24 -2m 19s Dec 25 -0m 11s
Apr 26 2m 14s Aug 25 -2m 2s Dec 26 -0m 40s
Apr 27 2m 24s Aug 26 -1m 45s Dec 27 -1m 10s
Apr 28 2m 32s Aug 27 -1m 28s Dec 28 -1m 39s
Apr 29 2m 41s Aug 28 -1m 11s Dec 29 -2m 8s
Apr 30 2m 48s Aug 29 -0m 53s Dec 30 -2m 37s
Aug 30 -0m 34s Dec 31 -3m 6s
Aug 31 -0m 16s



Negative values in the table indicate Dial Slow.  

Accuracy - Just how accurate are these estimates of EoT?
The above EoT figures are calculated using Meeus's 'simpler' formula (after Smart) and are generally accurate to within 4 seconds and to less than 1 second for most of the year.
To illustrate this, the graph below shows the differences in seconds between Smart's EoT figures for 2012 and those of the Great Circle data when a detailed comparison was made.  
NB A further Excel comparison made over the year 2014 (that is two years later in the centre of that leap year cycle) may be found Here (Needs Excel)

II - Correction for Longitude

When a dial is not on the local Time Meridian, for example for a dial in the UK that is not on the actual Greenwich Meridian, then the sun is overhead at the dial at a time earlier or later than at Greenwich.  The correction to be made is 4 minutes of time for every degree of longitude difference from Greenwich.  This figure should be subtracted from the dial reading for dials East of Greenwich (such dials are Fast) and added for dials West of Greenwich. (such dials are Slow).

III - Correction for Summer Time

Lastly during the period when Summer Time (Daylight Saving Time) is in force one hour should be added to the corrected sundial reading to obtain clock time.

Finally, how about a simple in-the-pocket card that allows you to check a dial at any Latitude or Longitude in the British Isles, by converting its Solar time to Clock time?

Click on the image below to find out how!

Click to download the Estimator


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