SUNDIALS


 

By way of Explanation!

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 essentially the same as Universal Time -UT) 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 are naturally only correct to within 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 to make 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

NOTE: If you have a WAP Mobile phone you can also access the Accurate EoT for any day in the next few months even whilst you are stood in front of an interesting dial and hadn't remembered to bring the EoT information with you! You do this by setting a bookmark into your phone and, whenever needed, you can call up the EoT for Noon GMT for any day in the next few months. The values are calculated using Jan Meeus' medium accuracy equations so giving EoT to within a second or two and often very much better than that. You should enter - all as if on one line - this rather complicated bookmark:

http://wmlproxy.google.com/wmltrans/h=en/g=/q=Sundials+horses/s=0/u=ourworld.compuserve.com@
2Fhomepages@2Fpatrick@5Fpowers@2Feot2.htm/c=0

Another advantage of using a WAP phone to obtain the information is that after retrieving the current month's EoT data it stays in your phone's cache so, if you don't use the GPS facility for other things, you can look up EoT figures all month instantly - and without the cost of a further call!!

After connexion to the WAP page you simply step down to choose the month. The displayed data shows the EoT in minutes and seconds (positive or negative as per the graph above) and adds the additional codes of (F) or (S) for Dial Fast or Dial Slow.

(Anyone having difficulty entering this bookmark may contact me to request sending it as a text message directly to their phone so permitting easier entry.)

 

 

A Large UK Millennium Dial

An interesting Millennium dial was opened in 2001 in Amble, Northumberland as part of a Town Square development.  

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 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:

If you would like to see something of the work of the British Sundial Society or how to record a dial for their Register, click Here.  We also record dials made in the UK but sold or sent overseas..  If you live outside the UK and know of any English or Scottish Dials in your country, please record them for us and send me a form and photo.  We have one UK Dial recorded as being in the Kyber Pass!  Can you do better than that?

Interested in horses too? - Go on, they're fun!!

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Last updated: 02/11/03