Astronomical rings  Astronomical rings (Latin: annuli astronomici), also known as Gemma's rings, are an early astronomical instrument. The instrument consists of three rings, representing the celestial equator, declination, and the meridian. It can be used as a sun dial to tell time, if the approximate latitude and season is known, or to tell latitude, if the time is known or observed (at solar noon). It may be considered to be a simplified, portable armillary sphere, or a more complex form of astrolabe. Parts of the instrument go back to instruments made and used by ancient Greek astronomers. Gemma Frisius combined several of the instruments into a small, portable, astronomical-ring instrument. He first published the design in 1534, and in Petrus Apianus's "Cosmographia" in 1539. These ring instruments combined terrestrial and celestial calculations. Fixed astronomical rings are mounted on a plinth, like armillary spheres, and can be used as sundials. The dial is suspended from a cord or chain; the suspension point on the vertical meridian ring can be changed to match the local latitude. The time is read off on the equatorial ring; in the example below, the center bar is twisted until a sunray passes through a small hole and falls on the horizontal equatorial ring. A sunring or farmer's ring is a latitude-specific simplification of astronomical rings. On one-piece sunrings, the time and month scale is marked on the inside of the ring; a sunbeam passing through a hole in the ring lights a point on this scale. Newer sunrings are often made in two parts, one of which slides to set the month; they are usually less accurate. In 1610, Edward Wright created the sea ring, which mounted a universal ring dial over a magnetic compass. This permitted mariners to determine the time and magnetic variation in a single step. The three rings are oriented with respect to the local meridian, the planet's equator, and a celestial object. The instrument itself can be used as a plumb bob to align it with the vertical. The instrument is then rotated until a single light beam passes through two points on the instrument. This fixes the orientation of the instrument in all three axes. The angle between the vertical and the light beam gives the solar elevation. The solar elevation is a function of latitude, time of day, and season. Any one of these variables can be determined using astronomical rings, if the other two are known.