SUNDIALS

Many people look at a sundial and think it is wrong!! 

So here, by way of explanation is the reason & EVEN a Presentation...

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 Solar or  '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.. Many countries operate Summertime (or Daylight Saving Time) as well.

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. The correction which applies around the globe is generally low during the period of the Northern hemisphere's summer.

• 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 and summertime corrections 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 you want to know a more Accurate EoT Correction for any day this year, Click the link to the left or here.  There is also a single page PDF for the whole year that you can print off.

* 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'. 

Text & Image © P Powers 2012-2023