Astronomical rings

History

The 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 instrument, astronomical rings. He first published the design in 1534, and in Petrus Apianus's Cosmographia in 1539. Gemma Frisius was known as a physician, mathematician, cartographer, philosopher, and instrument maker. He created important globes, improved the mathematical instruments of his day and applied mathematics in new ways to surveying and navigation.

Structure

The three rings are oriented with respect to the planet's axis and equator, and with respect to a celestial object.

When the shadow of the rings are aligned so that they appear to be in the same, or nearly the same, place, the meridian identifies itself.

Meridional ring

The meridian ring is hung vertically, with ring lying parallel to the local north-south meridian. A sliding suspension shackle is attached to the top of this ring, from which the whole device can be suspended. The ring is marked in degrees of latitude. When properly used, the shackle points to the latitude at which the instrument is being used. The whole ring is thus parallel to the circle of longitude passing through the place where the user is standing.

Because the instrument is often hung from the meridional ring, it is often the outermost ring, as it is in the traveller's rings illustrated above.

This ring can function as the gnomon, when the rings are used as a sundial. A horizontal line aligned on a meridian with a gnomon facing the noon-sun is termed a meridian line and does not indicate the time, but instead the day of the year. Historically they were used to accurately determine the length of the solar year.

On some forms of the astronomical rings, the meridian ring is engraved with arcs measuring 0-90 degrees on a latitude scale. At every 10 degrees there is the corresponding numeral printed along with a long has mark which is divided into even smaller units of measurement for a more accurate reading. A slightly smaller hash mark can be found at every half of the 10 degrees and even smaller hash marks note each degree. This allows for the measurement of the sun's height.

Equatorial ring

The equatorial ring occupies a plane parallel to the celestial equator, at right angles to the meridian. It aligns itself by being attached to the meridional ring at latitude zero. Often equipped with a graduated scale, it can be used to measure right ascension. On the traveller's sundial shown above, it is the inner ring.

This ring is sometimes engraved with the months on one side and corresponding zodiac signs on the outside; very similar to an astrolabe. Others have been found to be engraved with two twelve-hour time scales. Each twelve-hour scale is stretched over 180 degrees and numbered by hour with hashes every 20 minutes and smaller hashes every four minutes. The inside displays a calendrical scale with the names of the months indicated by their first letters, with a mark to show every 5 days and other marks to represent single days. On these, the outside of the ring is engraved with the corresponding symbols of the zodiac signs. The position of the symbol indicates the date of the entry of the sun into this particular sign. The vernal equinox is marked at March 15 and the autumnal equinox is marked at September 10.

Declination ring

The declination ring is moveable, and rotates on pivots set in the meridian ring. An imaginary line connecting these pivots is parallel to the Earth's axis. The declination "ring" of the traveller's sundial above is not a ring at all, but an oblong loop with a slider for setting the season.

This ring is often equipped with vanes and pinholes for use as the alidade of a dioptra. It can be used to measure declination.

This ring is also often marked with the zodiac signs and twenty-five stars, similar to the astrolabe.