Sundial Factoids

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

IT Sundial gnomon height
  • 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 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:
  • IT Sundial ad
    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. 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.
  • Delhi 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 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 turtlebay.org, atlasobscura, etc. None of these sources makes any attempt to document this claim. I have documented, in January and in June, that 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 Point of Infinity sculpture was installed on Yerba Buena Island in May, 2023. An article in the New York Times states that there will be "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". This overlooks the fact that every vertical structure in the northern hemisphere (above the Tropic of Cancer) casts a shadow that points due north (geographic north) at solar noon every day of the year, not just at the equinoxes or the solstices.
  • Ingleside Terraces Sundial links:
    Google map, directions to the Sundial
    Ingleside Terraces Sundial, Then and Now Photos
    Ingleside Terraces Giant Sundial
    Kite Aerial Photographs
    Sundial History
    Creative Commons License
    Robert Karis
    www.sfog.us
    Alphabetical index
    Extreme sundials
    Geographic north
    Ingleside Terraces Sundial
    Ingleside Terraces Sundial accuracy
    Sundial Bridge accuracy
    Sundial plaques
    Sundials EOT tables
    Winter solstice
    The North American Sundial Association has a registry of over one thousand sundials in North America, including the Ingleside Terraces Sundial, which is number 82.
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