Originally conceived by Gemma Frisius in 1534, it can be considered both as a portable, simplified armillary sphere, and also as an equatorial sundial.
Although known as the astronomical ring, it is principally employed as an equatorial solar universal quadrant. Its use was widespread during 18th century.
It is composed of a ring that can be adjusted to various latitudes, a meridian ring showing the latitude scale, a movable hour ring and a central, grooved bar that is graduated for the months of the year and a perforated cylinder, which moves over this. It will indicate the solar local time.
Unlike most sundials that require a compass to orient them North-South, this instrument can find the meridian when the shadows of the rings come into conjunction (that is, they overlap together).
Made of sturdy metal alloy and lacquered finish, it will not rust or corrode. The Astronomical ring comes with a wooden display stand and instructional booklet.
Diameter: 4 in.
Astronomical Ring diagram from:Johannes Dryander. Anulorum trium diversi generis... (Marburg, 1537)
Image credit: http://longstreet.typepad.com/thesciencebookstore/2010/09/visual-chronology-of-cosmologies.html, Public Domain, Wikimedia Commons.