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Sundial Time and Watch Time

To compare sundial time to watch time we need to know three things: (1) has the dial been designed, manufactured, and set up correctly, (2) the longitude of the place the dial is located and (3) the Equation of Time value for the day we're observing. Then we're ready to do some sums.

SUNDIAL TIME
If a sundial is to tell the time correctly, the layout of the hour lines on the dial plate and the angle of the gnomon must be calculated precisely for the lattitude in which it is to be located. A dial designed for Mizzen Head in Cork will not be accurate at Malin Head in Donegal and it must be properly set up i.e. the plate perfectly level in both directions with the gnomon pointing true north towards the Pole Star (NOT magnetic north)

LONGITUDE
The sun reaches its highest point above the horizon at noon at all places along the same North/South line of longitude. It follows that at that instant in places to the east of this location noon has already passed, and in places to the west of this location noon has not yet arrived. There is one hour of time difference for every 15 degrees of longitude difference. Clocks in Ireland are set to Greenwich (London)Time. When it is noon at Greenwich, in places in Ireland 7 degrees 30 minutes west of Greenwich it is 11:30 A.M.suntime.

THE EQUATION OF TIME (over simplified)
The earth does not move around the sun on a circular orbit at a constant speed, but rather in an elliptical orbit at a variable speed. The solar day varies from 20 secs less than the 24 hrs average day length we use for our clocks to 30 secs more. These small time differences accumulate over a period of months to reach a total of just over 14 minutes in mid-February, when "sundial time" is slow relative to "clock time", and to just over 16 minutes difference at the beginning of November, when "sundial time" is fast relative to "clock time". The following table, known as the Equation of Time, shows this accumulated difference in minutes for each day of the year. (in the 18th Century 'Equation' meant 'Correction').
(You will see the Equation of Time information in graphical form on a lot of sundials)
In the table below 24 hr days are shown as zero. When solar days are longer than 24 hrs: 16 April to 12 June and again 2 Sept to 24 Dec shown in red - Subtract.
When solar days are shorter than 24 hrs: 14 June to 31 Aug and again 26 Dec to 15 April shown in black - Add
When there is no value for a specific date, use the nearest value to that date







THE SUMS
Watch Time = Sundial Time + Longitude Correction ± Equation of Time.

Using the example of the photo taken by me on 10 August

Sundial Time  10:00
Long. corr. 00:29 ( my dial is located at 7° 18' 30" West )
Eq. of Time 00:05
Total

10:34
Plus B.S.T. 01:00 ( only needed in summer when our watches are set 1 hour "on" )

If you look real close my watch says 11: 34 The sundial and my watch agree.

If you don't like doing sums click here and my calculator will do them for you!


If you know the location of a sundial in Ireland (NOT a mass produced DIY Store garden ornament) please email it to me (Click here to email M.J.Harley) - a member of British Sundial Society

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