Sundial Bridge Accuracy

Sundial Bridge over the Sacramento River in Redding, California
The 217 foot high pylon points to geographic north. The pylon is angled at approximately 49 degrees from the horizontal, as can be seen in the photograph below. The length of the pylon is about 290 feet. The base of the pylon is located at latitude 40.593, longitude -122.3775. The pylon serves as the gnomon of the sundial. In a typical sundial, the gnomon's angle equals the latitude, so that the gnomon is parallel to the earth's axis Sundial Registry
Sundial Bridge Redding CA
The plaques on the Sundial Bridge use Pacific Daylight Time.  The shadow of the pylon points to geographic north at solar noon. The width of its shadow at the plaques is about 10 feet wide in winter and three feet wide in summer, due to the tapering of the pylon. As is true of all horizontal sundials, the left side of the shadow of the gnomon is used to read time until solar noon, and the right side of the shadow is used in the afternoon. The Sundial Bridge does not have precise markers, but time can be read with an accuracy of a few minutes.
The cell phone photos below were taken on January 22, 2022. Right click on the images to enlarge.
Sundial Bridge Redding CA
Sundial time 12:31 PDT, cell phone time 11:32 PST (12:32 PDT).
Contrast was increased in this photo to make it easier to see the border of the shadow.
Sundial Bridge Redding CA
Sundial time 1:59 PDT, cell phone time 1:00 PST (2:00 PDT).
In both of these photos, sundial time was about one minute before cell phone time.
This may be the largest sundial in the world with demonstrated accuracy.

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Robert Karis
www.sfog.us

Alphabetical index
Extreme sundials
Geographic north
Ingleside Terraces Sundial
Ingleside Terraces Sundial accuracy
Sundial factoids
Sundial plaques