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. These numbers are consistent, within an inch, with a right triangle with the angle of the gnomon equal to our latitude, 37.725 degrees (which is true for this type of sundial, so that the gnoman is parallel to the earth's axis).

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 12:00 sun time, solar noon.  This is true every day of the year. 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. The excellent accuracy of the Ingleside Terraces, within a minute on multiple dates throughout the year, is demonstrated in this page: Ingleside Terraces Sundial accuracy

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 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, and should have made URICO aware that the Ingleside Terraces sundial was _not_ the largest sundial in the world. 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.

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 seems impossible and has not been documented.

The Sundial Bridge in Redding, CA 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 atlasobscura, etc. None of these sources makes any attempt to document these claims. I have documented, in January and in June, that Sundial Bridge sundial time is within a minute or two of clock time: Sundial Bridge accuracy, which is as close as can be expected for a sundial without precise markers. The Sundial Bridge is located in a bend in the Sacramento River at Turtle Bay. TurtleBay.org also states that "The sundial shadow is not accurate in winter because the shadow is too far in the arboretum to be seen." Obviously the shadow can be seen in winter. The statment by Turtle Bay.org seems to be based on the idea that the tip of the shadow is used to read the time, similar to the way a clock is read using the tip of the hour and minute hands. That is not correct and would not work. If the tip of the shadow reaches the dial on the summer solstice (as it does in Turtle Bay), then of course as the year progresses the shadow gets longer and longer and the tip of the shadow projects far beyond the dial. This is true for all horizontal sundials. However, as my webpages demonstrate, time is read using the edge of the shadow, see Sundial Accuracy.

Point of Infinity, Yerba Buena Island, San Francisco

Point of Infinity Sculpture, 69 feet tall, on Yerba Buena Island, San Francisco. The San Francisco Arts Commission says that "The sculpture acts as a monumental sundial...A stone marker will be placed in the plaza to mark the precise location of the noon shadow on the spring and autumnal equinoxes." An article in the New York Times states that it is a sundial with "a granite marker on the ground to align with the shadow cast by the artwork at solar noon on days of the spring and fall equinox". These articles overlook the fact that every vertical structure in the northern hemisphere (above the Tropic of Cancer) casts a shadow that points due north (360 degrees, geographic north) at solar noon every day of the year, not just at the equinoxes or the solstices.

Despite the articles referenced on the left, the marker was not placed at 360 degrees, geographic north, as can be seen in the above photo. Latitude of the Point of Infinity sculpture 37.81047, longitude -122.36886. Spring equinox 3/20/25 12:19:10 PDT, 58 minutes before solar noon, the azimuth of the sun is 157.2 degrees, and the shadow reached the marker at 337.2 degrees NOAA. At the summer solstice, 6/22/25, the shadow pointed to the granite marker at 12:45 P.M. PDT but did not reach it. At solar noon, 1:11 P.M. PDT, the shadow was at 360 degrees (0 degrees), sun elevation 75.6 degrees, and the very short visible shadow did not reach the curb around the sculpture. On the autumnal equinox, 9/22/25, the azimuth will be at 157.2 degrees and hit the granite marker at 12:04:12 P.M. PDT, 15 minutes earlier than at the spring equinox.