There are a number of ways to tell the time with the sun but the most common one for most of us is the use of a Sundial. There are a number of factors that have to be taken into consideration if your Sundial is going to be close to the time you would see by looking at a clock or watch.
How many types of Sundials are there?
Sundial Co of the United Kingdom has an article on the internet where they identify seven different types of Sundials. The most common kind of sundial that can be found in use is the horizontal sundial. It is so defined because the sundial face is horizontal to the surface of the earth. The picture to the right displays a horizontal sundial that I have in my backyard.
Early uses of the Sun to tell time.
The Egyptians are believed to have used Obelisks as early as 3500 BCE to tell time. They used these devices first to divide the day into morning and afternoon and eventually around 1500 BCE into 12 equal parts a which defined the workday. A crude Sundial found in the Valley of the Kings is believed to date to 1500 BCE. Most historians give credit to the Babylonians for the invention of the Sundial but not until 430 BCE which is well after the example found in Egypt.
Can you tell time accurately with a Sundial?
If you set your Sundials correctly, they will tell time quite accurately. There are a number of issues involved in making the Sundial accurate and some of the issues require the use of tools that may not be readily available in your home. The pointed object on the Sundial that casts the shadow onto the sundial face is called the Gnomon (pronounced noumon). The angle of the Gnomon should be equal to the latitude where the Sundial is located. Using Google Earth, I was able to determine that my home is 47 degrees, 6’58.96 north of the equator. The tip of the Gnomon should, therefore, be at an angle of 47 degrees above the surface of the Sundial. To measure this angle you need a protractor. Some Sundials come with adjustable Gnomon but most like mine are at a fixed angle. Today most of the fixed Gnomon are at an angle of 40 degrees. As a result, the time will not be measured as accurately as a watch or clock.
The Impact of Daylight Savings Time.
The instructions for setting up your Sundial tell you that you should adjust the Gnomon so the shadow falls on the correct time at noon when the sun is at its peak height for the day. You have to read several instruction sites to find out that if you live in an area that uses Daylight Savings Time, then you adjust your Sundial at 1 PM which is really noon in sun time.
Location within a time zone.
The accuracy of your Sundial is further complicated by your location within your time zone. If you live on the eastern border of your time zone, the variation of sundial time from clock time will be different than the variation for a person that lives on the western edge of the time zone. For every one degree of longitude, there is a four-minute difference in the actual time the sun will be at its peak height for the day. To be completely accurate, you have to know how far east or west you are located from the center point of your time zone and adjust your sundial by four minutes for every degree of longitude.
A number of sources indicate there are four days in each year that are best for adjusting the settings on your sundial. They are April 15, June 15, September 1 (I have also seen September 2) and December 24. These sources indicate that adjusting or setting your sundial on other dates will result in the sundial being as much as 14 minutes behind clock time or 16 minutes ahead of clock time.
I think by now you would agree that making a Sundial tell time as accurately as a clock is more work than it is worth. Adjust the Sundial so it shows noon and just enjoy having a very old fashioned way of telling time in your yard.