Is it possible for a garden sundial to tell perfect time?
The chances are very good if you can make one yourself, or get one custom made for your location. But both these alternatives will take either time or money.
The reason for this is that apart from adjustments for time zones and daylight saving , covered in another article, there are three things that must be done.
Compensating for Earth's Path Around Sun
If earth followed a perfect circle as it revolved around sun, life with sundials would be easy. But its path is elliptical or oval, and this causes errors of up to 16 minutes in sundial time at some times of year. The corrections are straightforward, and can be made using a table, or from a figure 8 line called Analemma, often seen on old globes and sundials.
Correcting for Latitude
Let's consider components of a sundial for a moment. It consists of a dial on which time divisions, and sometimes other information is marked, and a triangular piece which sits vertically on dial. It is called gnomon, pronounced nomon, and part which casts shadow onto dial is called style.
For more information on how sundials work, including how to design your own, go to Google and type "how + sundial", without quotes but with +, in search box.
To be accurate, angle between triangular part of gnomon and horizontal must be same as latitude of place it is to be placed in (You can find latitude - and longitude - of your home from any topographic map or good atlas).The arrangement and distance between hour markings on dial must also be correct for latitude.
Hmmmm! This means that unless you are very lucky, that elegant sundial in your local garden supply shop will probably not show time particularly well. It may be calibrated for an average latitude (commonly 45 degrees), which is good if your latitude is not too different. Or it may be purely ornamental and will really only be useful around noon.
Now of course this doesn't matter at all if you are looking for something pleasing to eye, and don't mind answering inevitable question "Does it tell time?" But if you'd like your sundial to be more useful, make sure you find out which latitude it is calibrated to.
Once you know this, all you need to do to compensate is to work out difference, and tilt dial towards or away from due south depending on whether you need to add to or subtract from latitude sundial was designed for. There may be slight differences to ideal spacing of hour marks, but apparent time will be reasonably close.