In Defence of The Accuracy of Sundials

People frequently accuse sundials of being inaccurate; some more eloquently than others:

“I am a sundial

and I make a botch

of that which is better

done by a watch”   (Hilaire Beloc)

This is unfair and and is the result of ignoring two issues (or three, depending on the time of year).

1. Most obvious, but often overlooked; if it’s summertime clocks in Britain run one hour ahead of GMT (BST), at least for the moment. So remember to adjust for this.

2. Unless it has been calibrated otherwise a sundial will show local solar time, not Greenwich Mean Time, so for every degree it is located east or west it will be four minutes faster or slower than GMT.

3. The final issue to take into consideration, the “Equation of Time”, is the least understood; though this too is fairly straightforward if you know to adjust for it. This is the basic principle; the Earth’s passage round the Sun is not an exact circle; it wobbles a bit. So, for every day but four there is an adjustment to be made, and it can vary up to a maximum of nearly 16 minutes fast or slow. On four days in the year there is no adjustment to be made. This doesn’t mean it’s inaccurate the rest of the year. It just means you need to know what adjustment to make. Many sundials have Equation of Time graphs or tables as part of their markings, which makes this adjustment very straightforward.

One of the reasons why so many old houses have sundials was to set the clocks.