Sundials, to be accurate, must be designed for or adjusted to the particular latitude and longitude of the location. Necessary too is the 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.
The Sunquest sundial consists of a gnomon or shade casting device, and equatorial time-scale crescent, a crescent supporting the gnomon and time time-scale crescent called the latitude crescent, a pedestal, a base, a daylight saving time stop and longitude setting, and a gnomon adjuster for the solstice periods.
The Sunquest gnomon is related to the analemma, but differs from it in that the halves of the figure are separated and the ends have been stretched somewhat. Structurally, the curves for each half-year are placed at right angles to each other, with the apex of the angle opened to form a narrow slot. The halves of the gnomon are bent into compound curves so that their shadow-making edges are complimentary. The are almost, but not quite, asymmetrical since the sun-time to clock-time correction for the two half-years differ. When either half is turned to face the sun, a curved ribbon of sunlight passes through the slot, intersects the time-scale and corresponds with the Equation of Time for half of the year; the remaining six months being represented by the other half.
The portion of the curved slot passing the sun’s rays that meet the time scale changes from day to day and depends upon the declination of the sun. In the summer the sun is high in the sky and shines through the upper, or north end, part of the slot. The reverse is true in the winter when the sun selects an appropriate portion of the lower end of the gnomon slot, and offsets the necessary number of minutes on the time scale.
No reference to a graph or table of corrections, choice of scales, and mental additions or subtractions, therefore, are necessary; the Sunquest sundial corrects automatically for the early or late sun – though with less accuracy during two periods of the year. From about December 1 to January 15, the path of the sun across the sky remains without much change very close to its lowest observed path on December 22. During this period it varies as much as 11 minutes later to 9 minutes earlier than mean or clock time.
Correction is made by another scheme; using the date to bring into effect the applicable portion of the analemmic curve. The gnomon slot curve has been “stretched out” axially at the southern end, allowing it to show a more accurate departure on the time scale from the gnomon or sundial axis. The effective portion of the enlarged curve is brought into play by manually moving the gnomon adjuster to date marks on the underside of the gnomon. The north end of the gnomon has the same feature for the high sun path during several weeks before and after June 21.
It is a pleasant duty in the sunshine to make the date setting during the solstice periods, and twice per year to shift the equatorial crescent, along with the Daylight Saving changing of other timepieces, all by loosening one or two wing-nuts. The spectroscope-like line of light telling time between shadows is apparent even on slightly overcast days.
Text by Richard L. Schmoyer, 1983.