Gnomonic horizontal sundials (plus Post Update)

In my last blog post, I mentioned that relatively simple sundials such as “equatorial sundials” (including “bowstring equatorial sundials” as well as “globe- or spherical sundials,” distant cousins of equatorial sundials) and “gnomonic horizontal sundials” are better for making the Sun-Earth connection with K-12 students than more complex sundials. In this blog post, we will take a deeper look at “gnomonic horizontal sundials.”

Many, if not most people are familiar with the appearance of a horizontal sundial such as the one shown below. Typically, horizontal sundials have a triangle-shaped shadow caster mounted on a base plate.

Christ Church sundial | Alexandria, Virginia USA

In contrast, most people are unfamiliar with the names for the parts of a horizontal sundial.

The triangle-shaped shadow caster is known as the “gnomon.” The upper edge of the gnomon is the “style“; the “nodus” is a point located anywhere along the style, including the tip of the gnomon. The gnomon is mounted on a base called the “dial face/plate.”

Sunnymead Elementary School

A “gnomonic horizontal sundial” — such as the one located at Sunnymead Elementary School in Hillsborough, New Jersey USA — is a reduced horizontal sundial with a vertical gnomon.

In this case, “reduced” means all of the gnomon is removed except for the nodus — a single point along the style that is supported by a vertical pole. The nodus casts a shadow on the dial plate that indicates both the time of day and the date of the year.

The first two annotated images show the part of the gnomon that is removed, highlighted in translucent red.

Photo Credit: Chi-Lian Chiu.

Photo Credit: Chi-Lian Chiu.

Notice the hour lines radiate outward from the “dial center,” like the ribs on a Japanese fan, and the date curves run east-west. Time of day is read from the numbers above the upper date curve when Standard Time is in effect; read the numbers below the lower date curve when Daylight Saving Time is in effect.

As of 2007, Daylight Saving Time begins in the United States on the second Sunday in March and ends on the first Sunday in November. On the second Sunday in March, clocks are set ahead one hour at 2:00 a.m. local standard time (which becomes 3:00 a.m. local Daylight Saving Time). On the first Sunday in November, clocks are set back one hour at 2:00 a.m. local Daylight Saving Time (which becomes 1:00 a.m. local standard time). Source Credit: Daylight Saving Time, Astronomical Information Center, U.S. Naval Observatory.

The next annotated image shows the “analemma,” the odd looking figure eight located along the noon hour line (also known as the meridian line) on the dial face. In a nutshell, the analemma is used to correct Local Solar Time for Local Standard Time.

Photo Credit: Chi-Lian Chiu.

Look for more information about “date curves” and the “analemma” in a follow-up blog post.

Putting it all together, the last annotated image shows the nodus indicates the time of day is approximately 11:30 a.m. EDT on June 21.

NASS Sundial Registry No. 504 | Hillsborough, NJ | Sunnymead ES

The following resources are specifically related to the Sunnymead Elementary School gnomonic horizontal sundial.

• Sample calculations for constructing the Sunnymead sundial, using a rounded value of 40.5° N for the latitude, and a gnomon height of 100 cm (1.0 m).
• Here’s a link to a horizontal sundial template (PDF) for Sunnymead Elementary School, Hillsborough, New Jersey USA (40°31’53” N latitude). Instructions for assembly: Print the PDF; cut out the gnomon and dial plate; glue the gnomon to the dial plate.

EarthDial(s)

EarthDials are also “gnomonic horizontal sundials.” EarthDials? Yep. The EarthDial Project began in 2004.

The EarthDial Project is a partnership between The Planetary Society (the world’s largest space interest group), Bill Nye, the Science Guy® and Nye Labs, and Professor Woody Sullivan at the University of Washington. Source Credit: The EarthDial Project, The Planetary Society (archived by the Wayback Machine Internet Archive).

The FCPS/NOVAC EarthDial (ED-7) was located in Fairfax County (Alexandria), Virginia at the Thomas Jefferson High School for Science and Technology (TJHSST) Planetarium.

ED-7 | 38° 49′ N latitude, 77° 12′ W longitude

The following resources are specifically related to EarthDials in general, and ED-7 in particular.

• Sample calculations for constructing ED-7, using a rounded value of 39° N for the latitude, and a gnomon height of 10 cm.
• FCPS/NOVAC EarthDial Glossary
• How to Read the FCPS/NOVAC EarthDial
• Two motions — the rotation of the Earth around its axis, and the revolution of the Earth around the Sun — cause daily- and annual cycles in the Sun’s apparent path across the sky that can be observed indirectly using a horizontal sundial such as the FCPS/NOVAC EarthDial (ED-7). For details, see Observing Daily & Annual Cycles of the Sun.
• Earth’s Seasons – Equinoxes and Solstices, A One-Year Time Series of One-Day Time-Lapse Animations
• SCSA EarthDial Activity (teacher’s answer key available upon request)

Related Resources

• British Sundial Society Glossary (complete version)
• SCSA Pole-to-Dial Converter Calculator – Use the SCSA “Pole-to-Dial Converter-Calculator” to convert any vertical pole (e.g., a flagpole, utility pole, etc.) into a fully functional sundial featuring “date curves” (declination lines) for the equinoxes and solstices.
• Motions of the Sun Simulator – can be used for open-ended exploration of daily and annual motions of the Sun. Configure the simulator as shown in the following screen captures and click “start animation.”

Daily motion.

Annual motion.

Post Update

GIFfun.app (for MacOS) was used to create the following animation of screen captures from the “Motions of the Sun Simulator.”

Save a time-series of screen captures as PNG files; use Apple Preview to convert the PNGs to GIFs. Suggested GIFfun setting(s): Delay = 200 (or 500).

Copyright © 2023 Walter Sanford. All rights reserved.

Tags: Post Update, sundials