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.

  • Firstly, because the earth neither travels in a circle round the sun nor travels in the same plane as our equator, shadows cast on a sundial actually move at different speeds at different times of the year. Ordinary clocks cannot cope with this and for this reason a 'mean time' was devised to even out these variations over the course of a year. In the UK time is related to the longitude of Greenwich and this time is called Greenwich Mean Time or GMT (and which is now essentially the same as Coordinated Universal Time - UTC) but other places on the globe have their own times based on their own time meridian..

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.

  • Secondly, if the sundial is not located exactly on the local time Meridian (here in the UK,  it is the Greenwich Meridian), there will be a correction that needs to be made since sunlight 'arrives' earlier or later on dials that are East or West (respectively) of Greenwich. This correction is 4 minutes of time for each degree of longitude that the dial's location is away from Greenwich. A dial is slow compared with a clock when it is West of Greenwich and fast when it is to the East.

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. 
If however you want to know a more Accurate EoT Correction for any day this year, Click here.

* 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'. 

Variation of dial time correction

P Powers 2012, 2013, 2014


A Large UK Millennium Dial

An interesting Millennium dial was opened in 2001 in Amble, Northumberland as part of a Town Square development.   That development later won a prize!

Click Here if you would like to see it.

Interested to learn more about sundials?

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.

Some Links:

A page devoted to Information and Comment about BSS and Sundials generally

An independent web resource that may be of interest and use to BSS members and the public alike. Over 130 documents are now archived here by permission of their authors!

The BSS Register

Did you know that there is a
National UK Register of sundials?

More information


The British Sundial Society's website is at


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