https://www.precisionsundials.com/schmoyer.htm
A SUNDIAL WHICH KEEPS ACCURATE TIME
“The sun moves faster at some times than others (being eight days longer in the northern half of the ecliptic than in the southern)”
- so quoted Brewster in his “Lectures by James Ferguson” published in 1823.
Sundials, to be accurate, must, be designed for or adjusted to the particular latitude and longitude of the location.
Necessary too is a correction for the daily irregularities of the sun’s apparent motion if the dial is to read the very uniform, average, or mean hours and minutes by which clocks divide our lives.
Due to the combined eccentricity of the earth’s elliptical orbit and the tilt of the earth’s axis, sun-time throughout the year may vary from 14 minutes earlier to 16 minutes later than the mean, or clock, time.
Now, in addition to James Ferguson’s careful, and often referred to, analysis-of sun time-keeping, we must also correct for the Standard Time zones, and for Daylight Saving time.
The design of the Sunquest sundial is derived from the traditional armillary, with an axis parallel to the axis of the earth, and having a number of nested rings representing latitude, longitude, celestial equator, ecliptic, etc.
https://www.precisionsundials.com/original%20schmoyer%20manual.doc
The Schmoyer Sunquest is an equatorial sundial designed by industrial designer Richard Schmoyer of Landisville Pennsylvania around 1959 (for his original 23 page instruction booklet in Word format,
click here).
It reads civil time due to a clever analemma design of the gnomon.
From summer solstice to winter solstice the time is cast by a beam of light passing through the half-analemma on one side of the gnomon, and from winter solstice to summer solstice the time is cast from a half-analemma on the other side of the gnomon. For an exerpt of an article describing this in more detail from Scientific American, October 1959,
click here.
The dial is 15 inches in diameter, and 18 inches tall. It is machined and engraved from sand castings made from Mr. Schmoyer’s original wooden patterns.
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Editor Note:
For those of you who live in Reo Linda, as Rush Limbaugh use to say, a
gnomon isn’t a type of cheese.
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The Schmoyer Sunquest Sundial is accurate within 5 min a day.
Its the most accurate sundial I have ever seen.
You can’t buy this one any more.
To read time, the gnomon is rotated until the light beam forms a narrow slit. Dial is generally accurate to within 5 minutes.
It has a well thought out, technical dial.
Near the summer and winter solstices, the gnomon requires an additional adjustment based on the date.
This was to “stretch out” the flat portion of the analemma near the solstices, so that the gnomon can pass a beam of light that falls more perpendicularly across the time scale, enhancing readability.
The dial is adjusts for Standard and Daylight Saving time.
The time scale can be loosened and advanced or retarded by one hour depending on which side this adjuster is referenced (Standard Time in this photo).
The user’s longitude is initially fixed by a one-time setting of the adjuster to mean solar noon (mean solar noon is readily determined from the longitude, as explained in the owner’s manual. (This Model is different.)
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Schmoyer Sunquest article from Scientific American, October, 1959
https://www.precisionsundials.com/sunquest%20article/amateur.htm
The above article goes into much detail about how the Schmoyer Sunquest Sundial functions.
It is like a clock but no moving parts.
It can be adjusted for accuracy.
In The Amateur Scientist for August,” writes Richard L. Schmoyer, an engineer of Landisville, Pa., “you raise the question of why a man who owns an accurate watch and several clocks will go to the trouble of building a sundial.
Few will disagree with your conclusion that he is motivated in part by the intellectual charm of a device which, without moving parts, can convert the sun’s changing position directly into time.
But sundial-making holds other attractions for its enthusiasts.
In the course of developing a sundial one is exposed to a fascinating and well-defined mixture of mathematics, geometry, geography and astronomy.
The design o£ a sundial challenges our creative talents, and its construction puts our craftsmanship to an exacting test.
Finally, the designer who permits the primary time-telling function of the sundial to control its form adds spice to the project.
Hardware in pleasing though strange and unexpected shapes often emerges from the equations which describe the ever-changing slant of the sun’s rays.
Details of gnomon for sundial.
“These inducements led me to design a sundial last year which has become a continuing source of pleasure both to me and to my neighbors. With only a few simple settings during two seasons of the year the sundial can be made to indicate accurate clock time. It can be adjusted to the latitude and longitude of any point in the Northern Hemisphere, including those areas where clocks are changed for daylight-saving time. Clock time can be read from it to an accuracy of about one minute, even when the sky is covered by a light overcast.
“Most people find sundials attractive, so one must not altogether dismiss their ornamental properties The structure of my dial was derived from the armillary, a traditional form which continues to enjoy wide popularity. Those primarily concerned with the appearance of a sundial admire the geometric perfection of the armillary’s nested rings, representing latitude, longitude, tropics, celestial equator and the ecliptic.
Much the same pleasing quality is found, however, in the unsymmetrical crescent of the early and late moon. The armillary can be converted to this form by eliminating all except the rings representing latitude and longitude and opening these at one of the sides where they join at right angles. When tapered and strengthened, these rings become nested crescents, as shown in the accompanying illustration [Figure 3].
“The transformation from armillary to nested crescents demonstrates how a pleasing shape can emerge from a functional necessity. A good time-telling device should always fulfill its mission. The armillary falls short of this ideal.
During part of each day its pattern of ornamental rings casts shadows on the time-scale, which is carried on the inner face of the equatorial ring. Worse, in the seasons of the equinoxes (March 21 and September 28) the scale lies in continuous shadow because the plane of the ring then parallels the sun’s rays. By eliminating the useless rings and opening the functional pair into crescents the time-scale is exposed to the sun without obstruction.
Details of gnomon for sundial.
“The structure of a sundial which indicates clock time is simple in concept if not in the making. The crescents are supported at their edges by an arrangement of bolts, slots and clamps so they can be rotated in their respective planes. The latitude crescent is made in two parts with a flange at the inner end of each. Bolts pass through the flanges and through a slot in the longitude crescent. When the nuts are tightened, the assembly becomes a rigid unit. Similarly, the edge of one member of the latitude crescent is clamped between the jaws of a split pedestal which extends up from the base. By loosening a single wing-nut the whole assembly can be rotated in the plane of the latitude crescent and in azimuth.
“A pair of holes are drilled in the latitude crescent on the diameter which coincides with the axis o£ the equatorial crescent. These holes serve as bearings to support the gnomon. It is to the unique shape of the gnomon, which compensates for the effect of the eccentricity of the earth’s elliptical orbit and the tilt of its axis, that this sundial owes its property of keeping clock time.
If the earth followed a circular orbit around the sun, and if its axis were perpendicular to the plane of the ecliptic, the straight gnomon of the conventional sundial would indicate clock time.
The time shown by clocks is that of a fictional sun which leads the real sun by as much as 16 minutes or lags behind it up to 14 minutes, depending upon the observer’s location and the season.
This difference is known as the equation of time and is shown graphically as the analemma on globes, a closed curve in the form of a figure eight.
“The gnomon of my dial is related to the analemma but differs from it in that halves of the figure are separated and the ends have been stretched somewhat. Structurally the gnomon consists of a strip of cast metal bent at a right angle along its length.
The apex of the angle is opened to form a thin slot. It is supported at the ends by shafts which turn in the bearings of the latitude crescent. The halves of the gnomon are bent into almost symmetrical compound curves with respect to the long axis and are therefore complementary. When either half is turned to face the sun, the curved ribbon of light which passes through the slot corresponds with the equation of time for half of the year, the remaining six months being represented by the other half.
Time is indicated by the thin line of light, from the slot which falls on the time-scale between shadows cast by the halves of the gnomon.
“The portion of the curved slot through which the rays pass to the time-scale depends on the declination of the sun. In summer, sunlight falls on the dial at a high angle and reaches the time-scale through the upper part of the slot, where the curvature just compensates for a ‘slow’ sun.
The reverse is true in the fall, when the sun is low. The winter sun is also mostly slow, and in the spring the sun goes from slow to fast to slow to fast again. Whatever the season, the sun’s declination selects an appropriate portion of the curve to offset the equation of time.
“Some difficulty is encountered during the period from about December 1 through January 15, when the sun lingers close to its lowest path across the sky.
During this same period, however, it speeds up with respect to the fictional sun. A lag of some 11 minutes becomes a lead of about nine minutes.
The simultaneous change in declination is very small.
A similar event takes place in reverse during the weeks preceding and following the summer solstice on June 21, when the real sun falls behind the fictional one, again accompanied by little change in declination.
To accentuate the sundial’s response during these periods, the curvature of the slot is stretched out.
The gnomon must also be moved axially in its bearings, the amount of shift being determined by a stop on the shaft.
The adjustment is made by hand according to a scale of dates engraved on the gnomon, as shown in the accompanying detail drawing.
“The designing of the gnomon’ though tedious, is not difficult.
One first determines the rate at which a ray of sunlight moves across the time-scale.
This depends on the diameter of the crescent on which the scale is engraved and on the related distance between the scale and the gnomon.
Multiply 3.1416 by the diameter of the equatorial crescent and divide the product by the number of seconds in a day. In the case of a 13-inch crescent the result is .000473.
This number is used for computing the distance and direction by which the curved slot must depart from a straight line for successive weeks of the year.
This procedure can be illustrated by constructing a graph of the curve for one face of gnomon. First draw a straight line equal to the radius of the proposed crescent and erect a perpendicular of about the same length above and below one end of the line. The base line represents the sun’s mean elevation (0 degrees) on September 28.
Next, with the end of the base line as the point of origin, extend a line to the perpendicular at an angle of 21 degrees, 34 seconds above the base line. This represents the sun’s elevation on July 15.
Now make a similar angle of 21 degrees, 47 seconds below the base line. This corresponds to the sun’s elevation on December 1. Angles above the base line are regarded as positive and are designated ‘plus’; those below are considered negative.
Next, draw in angles at weekly intervals for all intermediate dates. A table showing the sun’s angular elevation throughout the year can be found in any ephemeris and in many almanacs.
These references also carry a table for the equation of time and list the difference between solar time and clock time in minutes and seconds.
The curve for one face of the gnomon is plotted from these values.
(If the thickness of the material from which the gnomon is constructed exceeds .01 inch, the curvature of the trailing edges must depart from that of the faces to avoid shadow.
The same basic procedure is used in computing all curves, however.)
For September 23 the equation of time has a value of -7 minutes, 35 seconds, which is equal to -455 seconds. Multiply this interval by the rate at which the ray of sunlight moves across the time-scale of the dial. In the case of my dial the computation is: .000473 X -455 = -.215.
This product represents the distance in inches by which the curve of one edge of the slot in my gnomon departs from the perpendicular.
(In plotting the curves all negative values are directed to the left of the perpendicular and positive values to the right.) The remaining points of the curve are similarly plotted for all intermediate dates at weekly intervals,
“The ends of the curve must be stretched out, as mentioned earlier. To accomplish this a perpendicular line is drawn through the point of origin and divided by a series o£ four points spaced a quarter of an inch apart both above and below the base line.
With these points as successive origins draw in the sun’s declination above the base line for the dates July 8, July l, June 26 and June 21 and below the base line for the dates December 7, 12, 17 and 22.
Similarly draw in the sun’s declination on the other half-face for June 1 to 21 and December 22 to January 15. The ends of the curves are then plotted from the equation of time by the method described.
The full-scale drawing is then ready for translation into hardware. All major parts of my sundial were cast in aluminum. The layout was drawn directly on the wood from which the patterns were made. The time-scale is divided into hourly intervals of 15 degrees each and subdivided into minutes as desired. The graduation representing noon lies in the plane of the meridian.”
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Captain Convey Sundial Comment
Did ya get all that bubba?
If you live in Reo Linda, as Rush Limbaugh use to say, don’t bother
reading this article about the Precision Sunquest Sundial.
Just buy a pocket watch or something.