Hooray for summer! Long days… glorious sunshine… the perfect time to think about sundials!
For centuries, sundials were the primary means of keeping time. The oldest known sundial was found in Egypt and dates from about 1500 BC. Sundials continued to dominate until the 19th century when mechanical clocks took over. Nowadays, sundials are more likely to decorate a garden or outdoor space than to serve as a timekeeping device. But, with careful planning and attention to detail, they can do both!
I’ve seen many sundials in my travels. Let’s take a closer look at a few of these classic instruments. This sundial, at Middleton Place in South Carolina, is an attractive garden feature. The sundial is displayed on a pedestal in a wide-open area where it can easily catch the sun’s rays.
This type of sundial is referred to as a horizontal sundial. It has a flat plate with an elevated, angled arm called a gnomon.
Hours are marked on the flat plate. The sun casts a shadow under the gnomon. The shadow hits the hour markings to indicate the time. According to the shadow on this sundial, it was close to 4:00 when I took this photo.
Here’s a less formal take on the horizontal sundial. The rustic wooden stand and minimalistic dial and gnomon are prefect for its Carolina Beach setting.
Many sundials, especially horizontal models, are inscribed with a brief, meaningful phrase. The sundial pictured below features the popular motto: “I Count Only Sunny Hours”.
A Book of Sundial Mottoes, compiled by Alfred H Hyatt and published in 1903, provides a variety of suggestions. Many of the mottoes were originally in Latin. A few examples are shown below. “Without Shadow Nothing” is my personal favorite.
Not all sundials have a flat surface. Armillary sundials, sometimes called shadow clocks, represent both a terrestrial globe and a celestial sphere. Armillary spheres are known to have existed in ancient Greece and China. Mechanical versions evolved to serve as planetarium devices for observation and teaching.
Although they are noticeably different from the horizontal sundial, an armillary sundial also keeps time by using a gnomon to cast a shadow on an hourly scale. The below armillary sundial was spotted in Chester, Nova Scotia. It is known as the Parade Square Sundial and anchors a small public garden. The sundial is huge—I’d guess it’s about 9-10’ tall—which allows for an unusually precise time scale with 5-minute increments. Notice the below photo was taken between 10:25 and 10:30.
Here’s another example of an armillary sundial. I spotted this one at the Keukenhof spring gardens in Lisse, Netherlands.
Hemispherium dials are yet another model of sundial. This one, from Brookgreen Gardens in South Carolina, is a charming sculpture by Brenda Putnam (1890-1975).
I believe the sundial pictured below would be called a bowstring equatorial sundial. It was also spotted at the Keukenhof.
This stunning artwork, spotted at Stichting Land Art Delft in Delft, Netherlands, is a wonderful, modern spin on the sundial. I’m guessing it would be considered a vertical sundial.
I pulled several different sundials into the below image for a side-by-side comparison. Models of a bowstring equatorial sundial, vertical sundial, armillary sundial, hemispherium dial and horizontal sundial are included.
Despite the remarkable differences, all of these sundials are timekeeping instruments—they all have markings to indicate the daylight hours and they all use the shadow of a gnomon to indicate the time. Chances are, I’ve tried to read the time on all of them—I can’t seem to pass a sundial without checking! More often than not, I compare the Solar Time on the sundial to the Clock Time on my watch, iPhone or camera. More often than not, the times are close but rarely are they exactly the same. I have always wondered why.
How to Read a Sundial
After considerable research and some serious puzzling, I think I have the full picture on reading sundials. A key point is that, because a sundial uses the moving shadow cast by the sun to keep time, the actual interaction between the sun and the Earth must be taken into consideration.
My most important finding is that the exact location (ie longitude and latitude) of the sundial impacts how the sun’s shadow will fall on the sundial. The sundial itself must be calibrated for the location’s latitude. Thus, a generic, off-the-shelf sundial cannot tell time reliably.
Step 1: Check the sundial for messages or markings that indicate it has been customized for the location. If so, find the shadow on the hour markings and take note of the Solar Time.
My second most important finding is that there are three calculations which must be applied to the sundial’s Solar Time to determine Clock Time. You may want to dust off your elementary school science skills for this next section! The first calculation is called the Equation of Time.
Equation of Time
. Sundials work because the Earth makes a complete rotation on its axis every day. The Earth’s axis isn’t perpendicular to the plane of the Earth, it is tilted at an angle of about 23.5°. The Earth’s orbit isn’t circular, it is actually an ellipse. The Earth’s tilt and its elliptical orbit cause the length of each day to vary slightly.
. Clock Time ignores this difference and is deliberately configured to treat all days as exactly 24 hours long.
. This causes a variance between Solar Time and Clock Time. Depending on the sundial’s location, it can vary from being about 14 minutes 20 seconds slow around 12th February to being about 16 minutes 23 seconds fast around 3rd November. Solar Time and Clock Time coincide on four days of the year: 16th April, 14th June, 2nd September and 25th December. The cycle repeats every year. The below graph illustrates the Equation of Time.
Step 2: Consult a detailed Equation of Time chart (or use an online calculator) to manually adjust the Solar Time by the number of minutes indicated for the relevant day of the year.
Longitude Correction for Time Zones
The second calculation is called Longitude Correction and it has to do with Time Zones.
. Clock Time is standardized into time zones. A single time zone covers a range of longitudes. Each time zone is identified by a specific reference meridian which lies midway between the east and west bounds of the zone.
. Solar Time is specific to the single meridian in which the sundial is located.
Step 3: Adjust the Solar Time to match what the reading would be at the time zone meridian. This is done by calculating the difference between the sundial’s longitude and the reference meridian of the relevant time zone. Then adding or subtracting 4 minutes for each degree of difference: subtract the minutes if the sundial is east of the reference meridian or add the minutes if the sundial is west of the reference meridian.
For instance, a sundial in my garden would be located at 83⁰ West in the Eastern Standard Time Zone. The reference meridian for this time zone is 75⁰ West. So, I would have a difference of 8 degrees, to the west of the reference meridian, and need to add 4 minutes for each of those 8 degrees. The result would be an increase of 32 minutes on my Solar Time.
Hours of Daylight
The third calculation is easier since daylight savings time is a familiar concept.
. A sundial is calibrated for either Standard Time or Daylight Savings Time.
Step 4: Determine if the sundial is set to Standard Time or Daylight Savings Time. Add or subtract an hour if needed.
Once again, I’m on the lookout for sundials. I’ll be searching for one which has been calibrated to match its location so I can try the Solar Time to Clock Time conversion. Admittedly, it will be much faster and easier to just check my phone for the time!
More Info
Check out the North American Sundial Society website for more information about sundials. The society is an association of people from a wide variety of disciplines who are interested in the study, development, history, and preservation of sundials. The website includes a directory of sundials in North America.
An interactive calculator designed to help compute the impact of the Equation of Time on a Solar Time reading can be found on the PLANETCALC website. See Equation of Time online calculator.
More information about the public gardens and towns mentioned in this post can be found at these linked websites:
. Middleton Place in Charleston, South Carolina, USA
. Town of Carolina Beach in North Carolina, USA
. Town of Chester in Nova Scotia, Canada
. Keukenhof spring gardens in Lisse, The Netherlands
. Brookgreen Gardens in Murrells Inlet, South Carolina, USA
. Stichting Land Art Delft park in Delft, The Netherlands
Today’s Takeaways
1. Sundials are the earliest known instruments for timekeeping.
2. Modern sundials can be both decorative and functional.
3. With careful planning and attention to detail, sundials can be used to determine Clock Time.
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