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 latitude 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
This site is copyright M.J.Harley ©