By way of Explanation...
As can be seen from this comparison of the times shown on a dial and on a radio clock, sundials usually don't show the same time as clocks. Clocks generally run consistently at the same speed and indicate 'mean time'.
A sundial on the other hand indicates what is called 'Local Apparent Time' - L.A.T. There are two main reasons why sundials show a different time to that shown on a clock or watch.
The difference between GMT & LAT is referred to by the rather quaint term 'The Equation of Time'*. This difference can be as much as 16 minutes in October/November and 14 minutes the other way in February.
Up to four times a year sun time and mean time come together and on these dates clocks and sundials agree - apart that is, from the longitude correction mentioned above. For dials actually on their own mean time meridian (and therefore not needing a longitude correction) these dates are April 16, June 14, September 2 and December 25. These dates may change by a day or so according to where the current date is in the leap year cycle.
The graph below roughly shows the 'Equation of Time' and it indicates
what sort of correction needs to be made throughout the year for the first
of the above effects.
* It is called the Equation of Time because in the Eighteenth Century the word 'Equation' was used in a different sense to that today. Then it meant 'Correction'.
©P Powers 2012, 2013, 2014
Try some light reading. A good starter book is: Sundials, their Theory and Construction by Albert E Waugh; Dover Publications New York 1973.
Another excellent book, this time made up of 9 paper cut outs, dials, sun compass, nocturnal etc - and some excellent explanations too - is Sundial & Timedials by Gerald Jenkins and Magdalen Bear. ISBN 0906212 59 6. Published by Tarquin Publications, Stradbroke, Diss, Norfolk IP21 5JP, England.
If you would like to see something of the work of the British Sundial Society or to see SunInfo - an independent news page about sundials, sundial societies, news, policies, comment and documentation; click Here.
BSS also record dials that were made in the UK but which were sold or have been sent overseas.. If you live outside the UK and know of any English, Scottish or Irish made Dials in your country, please record them for us and send me a form and photo. There is one UK Dial recorded as being in the Kyber Pass! Maybe you can do better than that?
Interested in horses too? - Go on, they're fun!!
Last updated: 31.12.12
© 2011, 2012,2013, 2014 Patrick Powers