Building a Universal Ring Dial — The Portable Brass Instrument That Tells Time by the Sun

A universal ring dial (also called an astronomical ring or universal equinoctial ring dial) is a portable brass instrument that tells solar time anywhere on Earth by projecting a pinhole of sunlight onto an hour scale inside a set of nested rings. Unlike a fixed sundial, which only works at the latitude it was designed for, the universal ring dial can be adjusted for any latitude — making it the traveller's sundial. The instrument consists of three concentric rings: an outer meridian ring graduated with latitude marks, a middle equinoctial ring graduated with hours, and an inner bridge piece with a sliding pinhole that adjusts for the Sun's changing declination through the seasons. When the dial is suspended from its top, adjusted to the local latitude, aligned to the meridian, and the pinhole positioned for the current date, a spot of sunlight falls through the hole onto the inner hour ring and indicates the solar time. These instruments were made by the finest instrument makers of the 16th-18th centuries, including Gemma Frisius who described the design in 1534. This blueprint builds a functional universal ring dial from brass strip, capable of telling time to within fifteen minutes at any latitude.