Walter Sanford's Blog

In a recent post about spherical sundials, I mentioned the sundial located on the grounds of Park Side Elementary School in Sebastopol, California. The Sebastopol Sundial is a combination sundial, featuring two types of sundials: a spherical sundial (also known as a globe sundial); and a “bowstring” equatorial sundial. The purpose of this follow-up post is to provide more information about “bowstring” equatorial sundials.

What is a “bowstring” equatorial sundial? Consider the following logical argument: 1) A spherical sundial, also known as a globe sundial, is a model of the Earth that is properly oriented to function as a sundial; 2) an armillary sphere is a reduced spherical sundial; and 3) a “bowstring” equatorial sundial is a reduced armillary sphere. Therefore, by deductive reasoning, a “bowstring” equatorial sundial is actually a reduced model of the Earth. For reference, see the following related resources:

1. 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.
2. Three photos of a bronze armillary sphere located at the University of California Berkeley. (The armillary sphere in Berkeley, CA was selected, in part, due to its proximity to Sebastopol.)
3. An annotated photo index of the Henry Moore sundial sculpture — a “bowstring” equatorial sundial located at the Adler Planetarium Sundial Plaza in Chicago, Illinois — explains how “bowstring” equatorial sundials work. See also an annotated photo that illustrates how a “bowstring” equatorial sundial is simply a reduced model of the Earth.

Close-up, Henry Moore sundial sculpture, Chicago, Illinois.

“Bowstring” equatorial sundials are typically set according to the following criteria:

• The gnomon or style (sometimes called a “bowstring” because parts of the sundial resemble a “bow and arrow”) — representing the Earth’s axis of rotation — is inclined at an angle equal to the latitude of the sundial, so that the gnomon is parallel to the Earth’s axis and the North Pole of the globe points toward the north celestial pole (Polaris, the North Star). For example, the gnomon of the Henry Moore sundial sculpture (see an  annotated photo) is inclined at an angle of 41.9 degrees (as shown by NASS member Fritz Stumpges’ SmartTool), same as the latitude of the sundial.
• The equatorial band (time scale) — representing the Earth’s equatorial plane — is inclined at an angle equal to the co-latitude of the sundial, so that the equatorial band is parallel to the Earth’s equator. For example, the equatorial band of the Henry Moore sundial sculpture (see an  annotated photo) is inclined at an angle of 48.1 degrees (90° – 41.9° = ∠48.1°).

Coming full circle, one aspect of the Sebastopol Sundial that makes it so appealing is its clever design. It’s reasonable to infer that the sundial designer intentionally chose to combine two types of sundials that are both similar and complementary: a model of the Earth (the spherical sundial); and a reduced model of the Earth (the “bowstring” equatorial sundial). Each type of sundial is simply a variation on a theme. (Repetition is a device used in architecture, art, music, poetry, and rhetoric.)

Editor’s Note: I currently serve as chairperson of the North American Sundial Society (NASS) Education Committee. Educators interested in exploring ways to use sundials to enhance and/or enrich classroom instruction are encouraged to contact me.

Posted from Arlington, Virginia, United States.

Now you see it; now you don’t. The gnomon, that is. Quoting my last post, “The Sunnymead Sundial features a nodus, but its gnomon is invisible.” The following annotated photograph (see a full-size version) shows the parts of the sundial you can see, as well as the parts that are invisible.

The Sunnymead Sundial, Hillsborough, NJ. Location: ≈40.5°N; ≈74.6°W.

Horizontal sundials typically feature a triangle-shaped gnomon. The upper edge of the gnomon, technically known as the style, is inclined at an angle equal to the latitude of the sundial so that the style is parallel to the Earth’s axis and points toward the north celestial pole (NCP). (Polaris, the North Star is co-located with the NCP.) All of the hour lines and polar-pointing style intersect at a point known as the dial center (or origin). The shadow of the style on the dial face is used to indicate the time of day. (For example, see a photo index of the horizontal sundial located at Freedom High School, Woodbridge, Virginia.)

The Sunnymead Sundial is a reduced horizontal sundial: All of the gnomon (refer to the area shaded in red, shown above) has been removed except for the nodus and its supporting pole; the shadow of the nodus on the dial face indicates both the time of day and the time of year (date).

Related Resources:

• Picasa Web Albums Photo Gallery of the Sunnymead Sundial (geotagged using Picasa 3.6); photos courtesy North American Sundial Society member, Chi-Lian Chiu.
• You can make a sundial similar to the Sunnymead Sundial. Use the Pole-to-Dial Converter-Calculator to convert any vertical pole (e.g., a flagpole, utility pole, etc.) into a fully functional reduced horizontal sundial featuring declination lines (“date curves”) for the equinoxes and solstices. For example, enter the following values for the Sunnymead Sundial: latitude = 40.5; and gnomon height = 1 (m). Use the Object Height Calculator to calculate the height of shadow-casting poles that are too tall to measure directly.
• The EarthDial Project
• Convince yourself that the Sunnymead Sundial is in fact a reduced horizontal sundial by constructing a horizontal sundial for 40.5°N latitude. The Horizontal Sundial Hour Line Calculator calculates hour line angles for a user-specified latitude; note that the results are consistent with the results from the Pole-to-Dial Converter-Calculator. (Compare the horizontal sundial you constructed manually with a template created automatically by You Can Make a Sundial.)

Editor’s Note: I currently serve as chairperson of the North American Sundial Society (NASS) Education Committee. Educators interested in exploring ways to use sundials to enhance and/or enrich classroom instruction are encouraged to contact me.

Did you know that some sundials, like the one at Sunnymead Elementary School in Hillsborough, New Jersey (shown below), not only tell the time of day but also the time of year? It’s true.

The Sunnymead Sundial. Photo credit: NASS Sundial Registry.

The gnomon is the part of a sundial that casts the shadow used to tell time. Some sundials feature another part — located along the gnomon — called the nodus; a nodus may be used to indicate both the time of day and the time of year (date). The Sunnymead Sundial features a nodus (shown above), but its gnomon is invisible. Huh? Yep, it’s true, but that’s a lesson for another blog post. In the meantime, remember the following mnemonic device: Notice the nodus! For this type of sundial, notice where the shadow of the nodus falls on the dial face in order to tell both the time and date — ignore the shadow of the pole itself.

On the dial face, declination lines (date curves) are shown for the equinoxes (March 21 and September 21) and solstices (June 21 and December 21); on these four dates, the shadow of the nodus will trace the declination lines. On any other day of the year, you can estimate the date by observing where the shadow of the nodus crosses the analemma around midday. Hour lines are marked for both standard time (along the “date curve” for December 21) and daylight saving time (along the “date curve” for June 21).

Editor’s Note: I currently serve as chairperson of the North American Sundial Society (NASS) Education Committee. Educators interested in exploring ways to use sundials to enhance and/or enrich classroom instruction are encouraged to contact me.