Subject: optical elements in dials

Date: Fri, 16 Feb 96 15:48 PST

From: xxxxxxx (Peter Abrahams)

To: sundial@rrz.Uni-Koeln.DE

>Does anyone know about dials... that use actual lens type optical systems?

Sundials with optical elements could have at least two functions: to focus the sun's rays and provide a narrower beam of light, and to allow a pointing accuracy far greater than the naked eye allows. An ingenious designer could use a concave mirror for these purposes, but using a lens would be a simpler construction. The scarcity of this type of dial could be because optical elements do not weather well, or because the accuracy of a sundial is limited by several considerations. One limit is placed by atmospheric refraction, which varies with the height of the sun in the sky, the altitude of the observer, and atmospheric conditions of pressure, temperature, and humidity; as detailed in the North American Sundial Society Compendium of 12/95. The diameter of the sun and the brightness of the sky cause any shadow to be indistinct, discussed in the Compendium of 8/94. Finally, irregularities in the rotation of the earth place a limit on the accuracy of a sundial, prehaps to an infitesimal degree; has this topic been addressed in the literature? A discussion of the theoretical limits of accuracy of a dial would be of great interest, and in fact was mentioned as a sequel to the 8/94 article, leaving me on the edge of my chair for some time now.

Other aspects of optical sundials are equally fascinating. Christopher Schissler's 1578 dial, now at the American Philosophical Society in Philadelphia, was the subject of an article in the Feb. 10, 1990 Science
News. This is a bowl sundial with an ellipsoidal inner surface, calibrated for two regions of latitude. When filled with water, the shadow of the gnomon (a bead on a string) is refracted to indicate one hour previous to the unrefracted shadow, a simulation of the story in Isaiah 38. Another optical dial is the polar clock of Charles Wheatstone, where the approximate time was indicated by the polarization angle of light from the sky. The previous post on the dipleidoscope is detailed in Gerard Le Turner's Nineteenth Century Scientific Instruments (for those without domestic sundial museums.) This meridian instrument held English patent 9793, June 20, 1843, by James Bloxam. It had a hollow 90 degree prism, and when the image of the sun reflected off the top glass coincided with the image from the two silvered sides, noon had occurred within a few seconds. One of the depicted models includes an optical sighting tube. Those who yearn for greater accuracy from sundial type instruments might well consider the coincidence of two reflected images. The dimmed image of the sun, reflected or refracted, gives a sharp disc to measure. Prehaps the next millenium will give us the first Hydrogen-alpha solar prominence sundial. Other examples will be the subject of future posts, by which time I will hopefully have found a terminology to describe sundials that use focused light, since all sundials can be described as optical instruments.

--'Peter Abrahams

xxxxxxxxx (Peter Abrahams)

the history of the telescope, the prism binocular, and the microscope