Sundial Factoids

factoid: An assumption or speculation that is reported and repeated so often that it becomes accepted as fact.

The 1914 Sundial booklet states that the gnomon of the Ingleside Terraces Sundial "rises to a height of twenty-six feet". However, as the above photo shows, with a 5'2" person standing to the next to the gnomon, the height of the gnomon is closer to three times the height of the person than to five times the height. There are webpages which also confuse the height and length of the gnomon. Measurements were performed many years ago with someone dropping a rope from the top of the gnomon, and the results were height of gnomon 17 feet, length of gnomon 28 feet. Using the actual latitude of 37.725 degrees, the height of the gnomon could be 17 feet and the length of gnomon 28 feet, within an inch.

There are several articles and webpages which state that the Ingleside Terraces sundial is synchronized with or calibrated to the summer solstice. For example, sfgate 4/14/2002, updated 1/30/2012, "Ingleside Terrace Giant Sundial": "synchronized to the summer solstice in June."  AtlasObscura.com "Urbano Sundial": "Synchronized to the summer solstice..." San Jose Mercury News 2/18/2020: "If you visit on the summer solstice in June, the time will be exact." The above statements are not correct. As demonstrated on other pages on this website, the gnomon points to geographic north which is at 12:00 sun time.  At the summer solstice, 12:00 sun time occurs at 1:11 P.M. Pacific Daylight Time.  On December 2-5, sun time and clock time are the same (within a minute) at the Ingleside Terraces sundial. Clock time can be determined from sun time by using the equation of time and the longitude correction as shown in the charts on this page: Sundials EOT tables

When the Ingleside Terraces sundial was built in 1913, there were many newspaper articles and ads stating that it was the largest sundial in the world:

Ad for Ingleside Terraces and "The Largest Sun Dial in the World", San Francisco Chronicle, October 4, 1913. The Ingleside Terraces sundial was most likely the largest sundial in the United States when it was built. However, there are larger sundials in India, built in the 1720's.

The Sundial Bridge in Redding has a 217 foot high pylon that faces north and which is also the gnomon for a sundial. Many articles state that the sundial is "exactly accurate on only one day in a year - the summer solstice" (inspired imperfection.com); or "The Sundial Bridges sole day of accuracy...the summer solstice" (redding.com), with similar statements in Wikipedia, turtlebay.org, atlasobscura, etc. None of these sources makes any attempt to document this claim. I have documented that in January the sundial is accurate within a minute or two: Sundial Bridge accuracy, which is as close as can be expected for a sundial without precise markers.

The Vrihat Samrat Yantra Sundial in the Jantar Mantar observatory in Jaipur, India, with a gnomon length of 164 feet, is the largest sundial in the world. There are claims on the web that it can be read with two second accuracy, which has not been documented.

The San Francisco Examiner, on February 23, 1913, contained an article with the following picture of the Vrihat Samrat Yantra Sundial in the Jantar Mantar Observatory in Delhi India. That article may have been the inspiration for building the Ingleside Terraces Sundial. The gnomon (hypotenuse) length of the Delhi sundial is 128 feet, and this sundial, with its approximately 100 stairs, is obviously much larger than the IT sundial.