Sundial Accuracy
Ingleside Terraces, San Francisco
Sundial Bridge, Redding
How accurate is a sundial?
The diameter of the sun as seen from the earth is about 0.5 degrees. As there are 15 degrees longitude per hour, light from the west side of the sun arrives about two minutes earlier than light from the east side of the sun. This limits sundial accuracy to about a minute or two.  But is the Ingleside Terraces sundial that accurate?
The photos below shed some light on this question.


sundial accuracy

This is IMG_20210123_115351231, taken, as its name indicates, on 1/23/2021 at 11:53:51. Suntime, as shown by the edge of the shadow in the photo, is 11:30 A.M. The blue-green index card increases contrast making it easier to see the edge of the shadow in the half-hour groove in the concrete. The NOAA correction for this date, at latitude 37.72468, longitude -122.46876, is 21 minutes, 52 seconds. As the leading edge of the shadow is used, another minute can be added to correct to the middle of the sun. Adding the correction to suntime gives 11:52:52, which is 59 seconds behind clock time on the cell phone.

sundial accuracy

One year followup: This is IMG_20220109_114908, taken on 1/09/2022 at 11:49:08. Suntime is 11:30 A.M. The NOAA correction for this date is 17'07". Another minute is added to correct to the middle of the sun. Adding the correction to suntime gives 11:48:07, which is 61 seconds behind clock time and is very close to the results obtained a year earlier.


sundial accuracy

This is IMG_20210119_134936701 taken on 1/19/2021 at 1:49:37 P.M. Suntime, as shown by the edge of the shadow in the half-hour mark in the concrete, is 1:30 P.M. The NOAA correction for this date and location is 20 minutes, 46 seconds. As the following edge of the shadow is used, a minute can be subtracted to correct to the middle of the sun. Adding the correction to suntime gives 1:49:46, which is 9 seconds ahead of clock time on the cell phone. In either morning or afternoon, the umbra, the darkest part of the shadow, is used to read suntime.

The same techniques can be used to check the Redding Sundial Bridge accuracy

CONCLUSION: The Ingleside Terraces Sundial is accurate within a minute for suntime in the middle of a sunny day.

sundial chart
Equation of time plus longitude correction for the Ingleside Terraces sundial.
EOT chart with two minute intervals
Numerical data from NOAA. This chart can be read within a minute.
For further accuracy when using a horizontal sundial, add a minute to the NOAA correction in the morning and subtract a minute in the afternoon, as explained below. Here is more detail about the equation of time.

EOT correction
This seven page article on DIAL and DIALLING from the Encyclopædia Britannica 1911 edition
(two years before the Ingleside Terraces sundial was built) explains the need for
one minute adjustments to the equation of time when using a horizontal sundial.

sundial accuracy

The Ingleside Terraces sundial has two metal rods at 12:00 and one at each of the other hours. The outer concrete curb has hourly, 15, 30, and 45 minute grooves; the inner concrete curb has hourly and half-hourly grooves. The colors have been retouched in this photo. Suntime cannot be read from this photo, as in the morning, the left side of the shadow from the gnomon is read; in the afternoon, the right side of the shadow of the gnomon is used.

sundial accuracy

This 1911 Scientific American article describes a sundial that was claimed to give correct standard time to the minute.

There are claims on the web that the Vrihat Samrat Yantra sundial can be read with two second accuracy, which has not been documented.
Accuracy and Precision: A set of data points can be said to be accurate if their average is close to the true value of the quantity being measured, while the set can be said to be precise if the values are close to each other Wikipedia. Neither has been demonstrated in photographs of the Vrihat sundial.

Creative Commons License
Robert Karis
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
Alphabetical index
Extreme sundials
Geographic north
Ingleside Terraces Sundial
Sundial Bridge accuracy
Sundial factoids
Sundial plaques
Sundials EOT tables
The North American Sundial Society has a registry of over one thousand sundials in North America, including the Ingleside Terraces Sundial, which is number 82.