Sundials

[-]  How to convert from sundial time to clock time:
(a) If it's daylight savings time (from the second Sunday in March to the first Sunday in November), add an hour to the sundial time.
(b1) Determine your latitude and longitude. To do that, use a GPS device, or in Google maps, right click on your location, click on "What's here?", and your latitude and longitude will appear. For the Ingleside Terraces (IT) Sundial, they are 37.725 and -122.47 degrees. (b2) The center of the time zones (clock time) are spaced at 15 degree intervals, i.e. 0 (GMT), -15, -30...-105, -120 (PST and PDT). IT is 2.47 degrees west of the center of our time zone, so our suntime is 60x2.47/15, approximately ten minutes earlier than our clock time, so ten minutes must be added to the sundial time.
(c1) A correction must be made for the "equation of time" (EOT). The earth does not travel around the sun at a constant speed. When the earth is near perihelion (closest to the sun, around January 3), the speed of the earth's revolution around the sun increases (Kepler's second law), solar noon appears later each day, and up to 15 minutes needs to be added to the sundial time to obtain clock time.  After aphelion (farthest from the sun, around July 4), up to 15 minutes needs to be subtracted to the sundial time to obtain clock time [eccentricity correction].

(c2) Around the equinoxes, March 21 and September 21, as the sun crosses the equator, suntime gets faster compared to clock time [obliquity correction].

(d)  The diameter of the sun as seen from the earth is about 0.5 degrees. As there are 15 degrees longitude per hour, this means that the shadow from the west side of the sun is about two minutes earlier than the shadow from the east side of the sun. This limits sundial accuracy to about a minute or two. The Ingleside Terraces sundial can be read within a minute or two, so having a longer gnomon or larger dial would not improve the accuracy of a sundial.
(e)  From September 27 to October 2 and from December 1 to December 5 in Ingleside Terraces, the corrections for longitude and for the equation of time cancel each other, although in September and October one hour must still be added to sundial time to obtain daylight savings time.
(f)  "Leap seconds" were not used until 1972 and are not taken into account in the equation of time. This may gradually make clock time slower than sundial time over a period of many years.
analemmaequation of time graph
Add ten minutes longitude correction to the times shown in the equation of time graphs to obtain Ingleside Terraces clock time, as explained in (b2) above.

The 1914 Ingleside Terraces Sundial booklet says that "on one side of the gnomon is set an exact table giving the minutes that must be added to or be deducted from sundial time in order to get true local time." That table is no longer present, but a new table, below, using data from NOAA, has been calculated: sundial chart
Equation of time plus longitude correction for the Ingleside Terraces sundial.
Sundial table spreadsheet and wallet-sized printout
For comparison, here is an equation of time chart. This chart corresponds to the equation of time graphs shown above. It applies to 0 degrees longitude and every 15 degrees removed from that. If you are 2.5 degrees west of the center of your time zone, add an additional 10 minutes to sundial time; 5 degrees west, add 20 minutes; 10 degrees west, add 40 minutes, etc. equation of time table
Numerical data from NOAA
Changes in latitude do not affect the values in these tables. Times are listed in two minute intervals so that the chart can always be read within one or two minutes, which is the approximate limit of accuracy of a sundial. From October-November to mid-February clock time gains 30 minutes on sun time as the earth is in perihelion.
Times in the IT sundial table are ten minutes greater than in the equation of time to correct for the fact that Ingleside Terraces, -122.5 degrees longitude, is ten minutes west of the center of our time zone at -120 degrees longitude.  Changes in longitude do not affect values in this table if you remain the same distance from the center of the time zone; e.g., -2.5, -77.5, -92.5, and -122.5 degrees longitude all have the same correction.
Use the left side of the shadow of the gnomon in the morning and the right side in the afternoon (assuming you are standing with the sun behind you).
To read the sundial to the nearest one or two minutes, wait until the edge of the shadow hits a mark in the concrete.  As noted in (d) above, the shadow from the west side of the sun is about two minutes earlier than the shadow from the east side of the sun.  To correct for this, add one minute to the NOAA correction in the morning when you are using the leading edge of the shadow, and subtract one minute from the NOAA correction in the afternoon when you are using the following edge of the shadow.  Shadows in concrete are not very sharp; try placing an index card across the edge of the shadow. Shadows are sharpest in the middle of a sunny day.  Ingleside Terraces Sundial Accuracy

[-] Sundial Bridge over the Sacramento River in Redding, California
The 217 foot pylon is aligned pointing due north but is not a true sundial (cf. sundials.org), as the pylon is angled approximately 49 degrees from the horizontal, as can be seen in the photograph below.  The base of the gnomon is located at latitude 40.593, longitude -122.3775.  To function as the gnomon of a sundial, the pylon's angle should equal the bridge's latitude, so that the gnomon is parallel to the earth's axis.
The Sundial Bridge is stated to be "exactly accurate on only one day in a year - the summer solstice, June 20 or 21."  The markers on the dial are positioned for Pacific Daylight Time on June 21.  Solar noon on June 21, 2018, for this location is at 13:11, at which time the sundial shadow points due north.  The equation of time (shown above) doesn't exactly apply due to the angle of the gnomon, but Sundial Bridge time would be expected to be accurate compared to clock time four times a year:  beginning of April, June 20-22, end of August, and end of December if an hour is subtracted to obtain Pacific Standard Time.  NOAA Solar Calculator  However, the Sundial Bridge does not have precise markers and the shadow of the gnomon is wider than the markers, so time cannot be read accurately using the Sundial Bridge. Sundial Bridge Redding CA
Sundial Bridge Redding CA
Sundial Bridge, Redding, CA

Creative Commons License
Robert Karis
www.sfog.us

Alphabetical index
Geographic north
Ingleside Terraces Sundial
Ingleside Terraces Sundial accuracy
Sundial Plaques