SF 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 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", which is due to the fact that 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, 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. (c2) Another correction is made for the tilt of the earth. Around the equinoxes, March 21 and September 21, as the sun crosses the equator, suntime gets faster.
Ingleside Terraces Sundial
(d) From December 2 to December 5 in Ingleside Terraces, the corrections for longitude and for the equation of time cancel each other.
(e) A final correction can be attemped for a one hundred year old sundial such as the one in Ingleside Terraces. For the first 60 years of this sundial's existence, 1913-1972, no corrections were made for "leap seconds".  It is difficult to read sundial time to less than a minute, but serial photos show that the Ingleside Terraces Sundial time is slower than but within a minute of the calculated time for solar noon.
analemmaequation of time
Kepler's second law

[-] Sundial Bridge over the Sacramento River in Redding, California
The 217 foot pylon is aligned pointing due north but it is not a true sundial, as the pylon is angled approximately 49 from the horizontal, as can be seen in the photograph. To function as the gnomon of a sundial, the pylon's angle should equal the bridge's latitude of 40.6, 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 using Pacific Daylight Time and the correction from the equation of time so that the shadow is accurate at the summer solstice. If you look at the equation of time (shown above), you can see that there are other days for which the Sundial Bridge time is accurate to within a minute: April 6-8, June 19-23, August 24-26, and December 28-29 if an hour is subtracted to obtain Pacific Standard Time  Time and Date.
Sundial Bridge Redding CA
Sundial Bridge, Redding

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Robert Karis
www.sfog.us