Sundials - World's Oldest Clocks

North American Sundial Society



Some years ago Bob Terwilliger built a Laser Trigon, an instrument that assists in drawing sundial lines on irregular shaped objects. For more about the Laster Trigon, see The Teacher's Corner - Sundial Projects. Here is the story of how he used it to create a bathtub sundial and after some years of service, how the sundial met its end.



The laser projects a beam of light to draw the lines and curves of a sundial onto any surface. To test the capabilities of his Laser Trigon, Bob decided to build a large combination vertical and horizontal sundial in his Florida home backyard … with some unusual consequences.  Here is his blog, taken from “A Dialist’s Notebook – The Shadow Garden

August 31, 1996 – Planning the Garden Sundial

 The Shadow Garden is an area in my back yard set aside for experiments in building sundials. The dials are transient, and all were made from found objects. Some are being built for fun - others to experiment with a method of construction.

terwilliger_garden_dial_modelInstead of putting a sundial in your garden, why not put a garden in your sundial? I am hoping the result [using the Laser Trigon] will be a fascinating and unpredictable garden of objects, some bearing hour lines, some numerals, some both, many neither. As the sun defines the day, its shadow will wend its way among them, anointing those it chooses with the power to tell time.

The first step is to make a working model. I need to get a feel for the relationship between the position of the gnomon and the west-facing board fence. The model consists of two planar dials, a horizontal dial joined to a direct west vertical dial. These surfaces will not be obvious in the finished dial, as the lines and curves will be located on the objects lying within the dial itself.

The Horizontal Dial is limited by the backyard space, such that only to the hours from10am-2pm will be seen, while the Vertical Dial to be attached to a north-south running fence will display the summer shadows to 8pm.

Next I installed the gnomon, which is made of fencing tubes and associated tee and elbow hardware. I temporarily located hour and half hour indicators on the ground by marking piles of whatever I could find.  The markers show the appropriate local solar time.

The hour line timing for this was done [with a now old] computer program, The Dialist's Companion, written by Fred Sawyer and myself for the North American Sundial Society. [Today a number of sundial calculators are available for your choosing, including:]

Orologi Solari by Gian Casalegnoterwilliger_garden_fence

zw2000 by Fer J. de Vries

Sonnenuhren by Helmut Sonderegger

Sundial Design by Miroslav Brož

A few of the objects I found to show local solar time (along with much pre-existing trash) are shown below the dial gnomon.

October 23, 1996 – Rethinking the Size of the Dial

I have to rethink this dial. If you take a look at the full size photograph, I think you will agree that the scale of the objects I used are too small for the size of the gnomon and the overall garden area.  I am going to have to find larger objects, or settle for a design that uses less shadow receiving surface.

February 21, 1997 - A Hot Tub is Found and Construction Begins

terwilliger_garden_plaintubI may have solved my problem. I found a derelict fiberglass hot tub … a discarded Jacuzzi bathtub… which has a suitable variety of surfaces and angles. Instead of many small objects, why not one big one? Tipped up on an edge, it fits almost perfectly between the posts outlining my desired dial area.

The hot tub has been tilted up facing south. It sat naturally in this position, but I provided further support so that I can walk on it without the tub moving. Drainage is provided by drilling holes in the apparent low points of the tub.

In preparation for realizing my dial, I tested the Laser Trigon on the tub. Sadly, the original laser was not bright enough to properly illuminate the surface. I have now acquired a much brighter laser, which I’m installing in the Trigon. The problem of mounting the Trigon has, in principal at least, been solved.

[Lasers have come a long way since 1997. Today, a number of 3-5mW (Cat-III) lasers are available for moderate cost.  Brighter Cat-IV lasers are available for under $1000]

March 4, 1997 – Drawing the Lines

terwilliger_trigon_photo_laserbeamThe new laser has been installed and it gives a perceptible dot. The mounting also works well, and I’ve already laid out the morning hour lines. I’ve scanned some Polaroid photographs to show the results.  The dial is being built for latitude 25° north, longitude 80° west.  My dial will include a longitude correction for the offset from the Eastern Time Zone at 75° west.

terwilliger_garden_trigonheadI’m using a sturdy tripod with a V rest on top and placed inside the tub as if to support the gnomon. Next, I’ll remove the gnomon and replaced it with the Trigon, which has been mounted on a length of the same tubing used to make the gnomon. The instrument and its tubing will be secured to the gnomon support post and rest on the tripod.  I’m including a photo of the Laser Trigon mounted on the tubing.

The instrument has replaced the gnomon and is positioned so the center of the axes of rotation is at the desired point of the nodus, a point that can be used to project the solstice limits of summer and winter and the mid year equinox line. The nodus point was somewhat arbitrarily chosen, but is close to the actual position of the nodus that will be determined more precisely by its shadow on the upcoming equinox. Another view of the process. Pepín tries his hand at the laser.

We start work at twilight, as the tub is too hot when in direct sunlight. This is our second session. Some of the lines have already been laid out and taped. I am directing the laser, Pepín is marking the position of the dot. The morning lines and three lines for declination are finished. The gnomon is back in place. The winter solstice and the equinox can be seen. The summer solstice is there, but it falls inside the tub. The lines are applied with 3M "Long Mask" masking tape. Hours and half-hours run through the tub. Quarter-hours are marked along the edges.


Interesting? It works for me.

March 15, 1997 - A minor setback

During a windstorm yesterday, a heavy extension ladder blew down and fell on the gnomon that fortunately had been removed and placed on the ground for safety!  The gnomon was bent and will have to be replaced.

April 3, 1997 – All the lines are finished

I have designed some numerals, which will be cut from vinyl by a signmaker who cuts custom letters with a plotter. The numerals will then be applied directly to the fiberglass. Gaps can be seen in the photo below for the numerals 1, 2, and 3. The time shown on the dial is just before 12:15 (the sharp eye will note - a bit after the equinox). The little numerals at the bottom are temporary.


Almost Finished

April 17, 1997 – The Finished Dial

After six days without sun, I was finally able to photograph the finished dial.  View from the south. The time is just before 11:45.  The flags are those of the State of Florida and the United States of America.


This calls for A Party. Click the audio panel below and listen to the Beatles' Here Comes the Sun.

March 20, 1998 - Performance of the Dial During its First Year

terwilliger_trigon_drawing_detailsThe hour lines were consistently accurate during the entire season. The taped lines themselves have faded slightly.

When making the Laser Trigon with my small clockmaker's machinery I encountered some problems laying out and machining the parts of the device necessary to produce the seasonal lines and curves. On the finished dial the winter solstice and equinox were reasonably accurate, the summer solstice less so. A bit of "tweaking" of the position of the nodus brought all the seasonal shadows within a half-inch or so of dead center. The results of the experimental dial were satisfactory, and some improvements to the seasonal adjustments of the Laser Trigon will produce an instrument which will, as advertised, "draw a sundial on a Buick".


April 28, 2000 – The End of the Hot Tub Sundial

The dial was completed in April of 1997. After 3 years in the direct sun, the dial has decayed. The masking tape lines have faded and washed away. I made an attempt to put vinyl tape over them, but it would not stick. Weeds have grown up through the drainage holes. Since the dial is in Florida I have filled it with concrete cylinders to prevent it blowing away during a hurricane. (I have always been amused by considering what an interesting UFO the dial would make should it fly away, and what the people who found it might think of it.)  Fortunate for all, it has stayed put.


Dial Completed in April 1997                                             Dial in April 2000

Installation and removal of the concrete cylinders during previous hurricane seasons had thrown the dial slightly out of alignment. It had never been secured to a foundation and I have always considered it temporary. The dial has survived three years without experiencing a hurricane, and I felt the odds might be against it surviving another.  In April, 2000 we took the hot tub dial down.


Going                                                        Going


--- Gone ---

The 3D Print Challenge

With all of Richard Schmoyer's SUNQUEST dial drawings available on the web, I decided that it would be a fine 3D printing project for the North American Sundial Society.  I use a filament 3D printer (Prusa i3 MK2) that heats  plastic, allowing the filament to be extruded into any shape.  The plastic I’ll use is Poly-lactic Acid or simply “PLA”, which is a biodegradable thermoplastic derived from renewable resources, such as corn starch in the United States and Canada.  Elsewhere PLA comes from sugarcane or even tapioca root. To make the PLA flow properly the 3D printer must be heated to 210 Celsius. 

So the challenge is to use the 3D printer to create parts that matched Schmoyer's detailed drawings suc as the equatorial crescent shown here at the right.

The Modeling Process

To render this drawing into a 3D sundial part, I use OpenSCAD, a public domain application for numerical computer aided design.  The language is reminiscent of Visual Basic. But it focuses on creating and manipulating fundamental 3D basic shapes such as cubes, cylinders, and polygons under programmatic control. There is a set of translation and rotation operators for placement of the objects and Boolean operators for concatenating or subtracting one object from another. 

To create the equatorial crescent ring I used two cylinders of slightly different size with vertically offset centers.  A boolean operator takes the difference between the two cylinders, leaving the crescent.  Other booleans drill out the slot, leaving a 3D stereolithographic file (STL) that conforms nicely to Schmoyer's drawing. The slot will allow the crescent to be rotated back and forth on the sundial’s meridian ring.  This adjustment allows for both longitude correction of time and daylight savings time.  As you’ll see in a moment, Schmoyer uses the gnomon itself to adjust for the equation of time.

Other major components are the meridian quadrants that fit around the equatorial crescent and the base and pedestal.  Once again the original Schmoyer drawings from nearly 60 years ago provide the details of how to make these replica parts.  Since the dial is scaled, certain modifications are required such as the mounting brackets that glue onto the meridian quadrants.  Likewise, I decided to make the pedestal as a single object instead of the two that Schmoyer laid out.  Because of the size of the meridian quadrants and pedestal, a single bolt holds them together. The result of OpenSCAD design process is a stereolithographic object, also called an STLfile.
OpenSCAD design of meridian quadrants, base, and pedestal

The Slicing and Printing Process

Once I've designed SUNQUEST STL files, they need to be prepared for 3D printing. This step is called “slicing”.  Here I use a program called Slic3r to take the STL object and decompose it into many horizontal layers, like pages of a book.  These layers will be printed sequential by my 3D printer, ultimately forming layer by layer, the STL object in plastic. There are a number of slicing programs such as “CURA” , “Slic3r”, “CraftWare”, Netfabb, Rep-et-ier, and Simplify3D.  After experimenting with a number of these programs, I prefer Slic3r. That’s “Slicer” with letter “e” turned backward in hacker format using the number 3.  Here is an example of two of the many layers that make up the Equatorial crescent and a photo of it being printed.

Sliced Equatorial ring and 3D Printer at work

The interior of the dial could be solid, but it is more efficient to select an infill pattern like a honeycomb that uses only 20 percent of the volume.  Each layer is 150 microns thick.  This is a compromise between fast printing that uses a 200 micro layer and wanting a very smooth, detailed surface using a layer as thin as 100 microns  Assuming the same speed of extruder nozzle movement, the difference between layers made at 200 vs. 150 microns is the ratio 4/3 and saves 3D print time by one half hour.  Even so, to print the Equatorial cresent, meridian quadrants and their braces takes about 5 hours of 3D print time.  I've tried increasing the speed of the 3D printer but usually something happens that spoils the final object.  So I have had to learn patience.

Gnomon Design and 3D Printer making gnomon

The most difficult part to make is the gnomon.  For most equatorial and armillary sundials the gnomon is a simple rod casting a small shadow onto the equatorial time ring.  But Richard Schmoyer took the shape of the sun's apparent seasonal motion in the sky (called the "analemma") and embedded it into the shape of the gnomon. As Schmoyer himself explained: "Time is shown, not by a shadow, or by one edge of the [gnomon] shadow as in the familiar garden sundial, but rather by a band of sunlight between two shadows cast by the gnomon onto the [equatorial crescent] time scale."    “The gnomon is turned by hand on its axis to sharply define a band of sunlight illuminating the time on the equatorial hour ring. “

“But in reading the sundial, when either the winter to summer or summer to winter gnomon face is turned toward the sun and gradually brought to a position at right angles to the sun's rays, two things happen:

(1) "The effective slot width is reduced or pinched down, making a more narrow line of light fall across the time scale. Turning beyond a right-angle shuts out the direct sunlight entirely, but just short of a position squarely facing the sun, the band can be as fine as you care to see and use to interpolate between the [time mark] graduations."

(2)  "The band of sunlight is shifted from the gnomon axis to fall earlier or later on the time scale by an interval necessary to show standard clock time instead of local sun time." This correction is known as the 'equation of time' and is made by the curve of the SUNQUEST gnomon."

Most important in this gnomon design is that it converts solar time back into civil time.  The Schmoyer SUNQUEST dial allows you to read the time that corresponds to your wristwatch!

In November,1958 Richard Schmoyer created a detailed drawing of the gnomon showing the curve displacement for every week during the year. This was truly a monumental calculating and drafting effort.  The analemma shape is accurately represented in the middle of the gnomon, but as one gets near the solstices, the analemma is physically stretched a bit to accommodate the rapid change in the equation of time.  In his dial, Schmoyer allows “The effective portion of the enlarged curve [to be] brought into play by manually moving the entire gnomon axially north or south."

Rather than copy all of Schmoyer’s numbers to create the gnomon shape, I took the polynomial approximation to the equation of time (EOT) and used the sun’s declination to find the scaled gnomon displacement.  This was probably the hardest part of designing the dial, since it forced me to make the gnomon of very, very thin rectangles… one for each 12-hour time span.  These rectangular layers are like brickwork that are moved laterally to create a slight overhang and moved forward and back for the appropriate EOT displacement all according to the sun’s declination.  Think of it as building a wavy wall one layer at a time, but each layer is perfectly straight.

Schmoyer's Drawing of the SUNQUEST gnomon


One of the highlights of the NASS conference in St. Louis was the 3D printed Schmoyer Sunquest civil time dial rendered by Bob Kellogg and featured in the NASS Compendium.  Following a video on his 3D printing process, NASS gave away 3 dials he had donated.  That left a roomfujl of people wondering how they could get one!

Since that time, Bob has made a number of tweaks to the design - including provisions for a dial in the Southern hemisphere and adding a knob to move the gnomon precisely for the weeks around the solstice where the gnomon Equation of Time has been stretched

Schmoyer Model Kit and Finished Dial

The Schmoyer Sunquest is a 3D printed 1/3 - scale plastic version (5 1/2" diam.) of Schmoyer's original  design.  It comes as a kit that is easily assembled and adjusted for your location with a Phillips-head screwdriver and about 5 minutes of your time.  Full instructions are included.

So now it's time to make the dials gernerally available.  We are happy to announce that you can now purchase the Schmoyer Sunquest from NASS (all profits will benefit NASS goals of education and fostering dialing projects and the art of gnomonics as a 501(3)c not for profit organization).  Bob has volunteered to provide the society with a supply to meet the demand.  But note that he can only print about 1 dial per day! -  so get your order in.

Price: $35 US dollars each plus s/h

Shipping/Handling (s/h) in: US  $7 for 1-2 dials
  Canada/Mexico $25 for 1-2 dials
  Elsewhere $34 for 1-2 dials

Please make your U.S. dollar denominated check (drawn on a U.S. bank) payable to NASS.  Provide your mailing address information and send payment to:

Fred Sawyer, 27 Ninas Way, Manchester CT 06040 USA

Alternatively, you can use PayPal to send payment to This email address is being protected from spambots. You need JavaScript enabled to view it.. (Note: Payments from outside the U.S. can be made in U.S. dolars via PayPal.  PayPal will do the currency conversion for you.)

All requests will be filled in the order in which payment is received.  If you want a dial for the Southern hemisphere or the original "classic" Schmoyer Sunquest dial without adjustment knob, you must let us know at the time you order the dial. 

This Sundials for Starters appeared in The Compendium in December, 2005

by Robert Kellogg, Ph.D.

Dial and Gnomon Set to Correct Site Latitude of 38 Degrees

This is the start of a regular column to review the basics of Sundials.  Of course NASS provides an introductory CD disk on sundials and there is always the classic reference Sundials, Their Theory and Construction by A.E. Waugh.

For this article, let’s consider some basics in buying a sundial for the garden.  Or perhpas you want to make one.  There are magazine catalogs and websites that offer “fine English dials”.  But are they for you?  Consider the latitude of an English dial.  London is at about 51º north latitude, while most of the populated area of North America is below 45º.  Take a look at the gnomon of your potential purchase. The gnomon should be approximately the same angle as you latitude.  Here we’ll illustrate a dial (Figure 1) from the Naval Medical Center in Bethesda, Maryland with latitude of approximately 38º.

Download this file (SerlesRuler.jpg)Serle's Ruler[Ruler to Measure or Make a Sundial]37 kB

Read more: Sundial Latitude

He may be wheelchair bound, but that doesn't diminish Tom Laidlaw's enthusiasim for sundials.  In front of his house on Carolina Lane is the Vancouver Heights neighborhood landmark - a sundial garden.  And what has he planted?

There is a bright circular equatorial sundial that shows the time from 4am to 8pm (and even an offset for daylight saving time).  On the grass is an analemmatic sundial sundial marking time from 6am to 6pm for anyone who wants to stand to the plywood walkway.  On a table near the house are a series of globe, equatorial and horizontal sundials as well as other sundial types that he will gladly explain.  For example, Tom has turned a skate board into a polar dial by adding a "T" gnomon in the middle.  And then there is a model of the Jefferson dial where you swing the gnomon around a globe to cast only a thin line shadow

Katie Gillespie, of the Columbian, reports "The 80-year-old retired electrician has always been a 'do-it-yourself kind of guy,' he said. For a while, it was skateboards he fancied, and bookshelves, and a Benjamin Franklin chair that transforms from a chair into a stepladder. He’s self-taught, he said, researching new projects online, then diving in.... 'It’s fun to watch him talk to people about it,' said Debra Brouhard, Laidlaw’s daughter and neighbor."

His latest obsession is sundials.  As a member of the North American Sundial Society, Tom now designs a multitude of sundials.  Visitors see his yard dotted with all types of sundials.  They come in all sizes: big and small.  His analemmatic sundial on the lawn always draws attention. Nearby, a plumb bob dangles from a beam. allowing Tom to tell time solar noon. when the shadow draws a line on the lawn pointing due north.

Gillespie found that, "Laidlaw’s passion for sundials began in 2009, when his grandson, Doug Brouhard, stuck a stick in the ground while they were camping. Doug Brouhard was about 12 at the time, and the dial didn’t quite work, Laidlaw said. It was the right idea, though, and a new hobby was born. 'I still have the stick that started it all,' Doug Brouhard said."

Read more of Katie Gillespie's article and see more photos of Tom Laidlaw and his sundials at

Read The Seattle Times article of May 14, 2013:

nass_news_2013_may_Sullivan_DialIn the May 14, 2013 edition of The Seattle Times University of Washington professor Woody Sullivan was honored as ‘Mr. Sundial’ for his persistence in declaring Seattle as the Sundial Capital of North America. Dr. Sullivan has worked on more than a dozen dials in Seattle, including the large 20x30 foot vertical dial on southwest wall of the Physics and Astronomy building at UW and the elegant 11x17 foot reflection sundial painted on the ceiling of his garage at N 47° 41.232, W 122° 21.562.  A small circular mirror outside the south facing window reflects a spot of light onto the ceiling.

The reflection sundial was a labor of love taking over 3 years to create.  Woody marked over 700 locations on the ceiling that allowed him to draw the local solar time, dates, hours of daylight, solar azimuth and altitude, analemma, and even hours to dawn.  And being a radio astronomer, he marked the transit sidereal time for two radio sources.  The dial was painted with marvelous beauty by a local mural artist, Jim Noonan.  The sundial is very personal to Woody, showing time marks for the date he married to the birthdays of himself, his wife, and two daughters.  The zodiac painted along the ecliptic has a local touch, representing Pisces by a pair of sockeye; Cancer by a Dungeness crab; and Capricorn as a mountain goat. There is even a compass rose.

nass_news_2013_may_UW_dialFred Sawyer, president of the North American Sundial Society (NASS) is quoted by Erik Lacitis, Seattle Times staff reporter, saying “it’s likely true that Sullivan’s garage sundial is one of the best in North America.”  When the NASS visited Seattle for the 2011 annual sundial conference, they visited Woody’s Reflection Dial and a marker was added to it for the date and time of itsofficial dial dedication held on Saturday 22 August 2011. “We toasted the sundial with an appropriate wine, ‘Wehlener Sonnenuhr’ by Joh. Hos. Prum, the fine Mosel Kabinett from the German vineyard with a large vertical sundial.” said Roger Bailey, the society’s secretary.

Among Dr. Sullivan’s accomplishments in the world of sundials began in the early 1990’s when the University of Washington’s Physics and Astronomy Building was being designed.  He suggested a large vertical (southwest declining) sundial.  The dial was completed in 1994 and Woody was hooked. He also helped design sundials used on the Mars exploration rovers Sprit and Opportunity that landed ion the Martian surface in 2004.  A campaign to build sundials all around our world ensued with the motto “Two Worlds One Sun”.  [photos from NASS]


  • Sundials for Starters
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  • Conferences
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  • Sawyer Dialing Prize
    Fred Sawyer, in cooperation with the North American Sundial Society, established a continuing yearly award, the Sawyer Dialing Prize to be presented by NASS to an individual for accomplishments in or contributions to dialing and the dialing community.
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  • Terwilliger Sundials
    In these pages is the famous tub sundial created by Robert Terwilliger using his laser trigon to lay out hour lines on a very irregular surface to create a working sundial.
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  • Biographies

    Who are today's sundial artisans?  Here are several bioghraphies of several artisans that show the unique combination of talents in art, engineering, and mathematics.

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  • Sunquest Sundial

    This section is dedicated to Richard Schmoyer who invented the Sunquest sundial.  Please visit as well.

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