Planet Earth is a magnificent timepiece! In a very real sense, our planet is a sundial. As the Earth rotates, specific times of day are indicated by the passing of light and shadow: the terminator marks dawn and dusk; the subsolar point marks midday, or local solar noon. At this point you may be wondering, “Since a globe is a model of the Earth, could a globe be used as a sundial?” The answer is, “Yes, of course.”
A spherical sundial, also known as a “globe sundial,” is a model of the Earth that is properly oriented to function as a sundial. Spherical sundials are typically set according to the following criteria:
- The globe’s axis of rotation is inclined at an angle equal to the latitude of the spherical sundial, so that the globe’s axis is parallel to the Earth’s axis and the North Pole of the globe points toward the north celestial pole (Polaris, the North Star). Editor’s Note: The axis of a globe is usually tilted 23.5 degrees from vertical, the same as the tilt of the Earth’s axis.
- The globe is oriented so that a specific location — for example, the place where the spherical sundial is located — is on top of the globe, so that the location points toward the nadir (opposite from the zenith). In this orientation, the line of longitude that passes through the location functions as the 12 noon hour line on the sundial.
Spherical Sundial located in Sebastopol, California.
The Sebastopol Sundial (shown above) is a combination sundial, featuring two types of sundials: a spherical sundial (also known as a globe sundial); and a “bowstring” equatorial sundial. The sundial also features a large analemma that is badly weathered. Two noduses — the pointed ends of the rod that passes through the globe — indicate the time of day and the time of year (date). Hour lines- and declination lines (a.k.a. “date curves”) for the equinoxes and solstices are marked on both the “equatorial band,” also known as the “time band” (below the globe), and the concrete pad for the sundial (at ground level). The analemma is marked along the meridian line (noon line) on the concrete pad.
The sundial is located on the grounds of Park Side Elementary School, at 38° 24′ 9.6″ North latitude, 122° 49′ 36″ West longitude (as determined by examining a geotagged photo taken with the built-in camera of an Apple iPhone 3GS). According to the North American Sundial Society Sundial Registry, the sundial is set incorrectly for a latitude of 40 degrees — close to the true value, but the difference is enough to affect the accuracy of the sundial.
A closer look at the globe shows that it is oriented so that Sebastopol, California is on top; the meridian line (line of longitude) passing through Sebastopol (≈122.8° W) is the 12 noon hour line on both the globe and the sundial.
- Annotated photos of the Sebastopol Sundial showing the location of Sebastopol, California on the globe and the major parts of the: combination sundial; “bowstring” equatorial sundial; and analemma.
- Picasa Web Albums Photo Gallery of the Sebastopol Sundial (geotagged using Picasa 3.5); photos courtesy Cecelia Ceccone. See also, a photo index of full-resolution original images (not geotagged).
- Another Picasa Web Albums Photo Gallery of the Sebastopol Sundial (geotagged using Picasa 3.5); photos courtesy Laurie Whiteside, Principal, Park Side Elementary School. See also, a photo index of full-resolution original images (not geotagged). Editor’s Note: The date and time shown in the EXIF info for each photo is incorrect. According to Ms. Whiteside, “The photos were taken on 30 November 2009, mid-morning, around 10:00 a.m.”
- And another Picasa Web Albums Photo Gallery of the Sebastopol Sundial (geotagged using Picasa 3.6); photos courtesy Laurie Whiteside, Principal, Park Side Elementary School. Editor’s Note: The date and time shown in the EXIF info for each photo is incorrect. According to Ms. Whiteside, “All [of the] photos were taken at approximately the same time on the same day, Wednesday, December 23, noon(ish).”
- “Sebastopol School Sundial” Waymark
- Thomas Jefferson’s Spherical Sundial. See also, Replica of Spherical Sundial Installed. Jefferson’s spherical sundial is a globe sundial with a movable vane, technically known as a shadow plane sundial.
Editor’s Note: I currently serve as chairperson of the North American Sundial Society Education Committee. Educators at Park Side ES interested in exploring ways to use the Sebastopol Sundial to enhance and/or enrich the curriculum are encouraged to contact me.