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. The altitude of the sun does not change much in a single day at the poles (where the sun rises and sets once a year), so rough measurements of solar altitude don't vary with time of day at high latitudes.
Use as a calendar sundial When the
solar time is exactly noon, or known from another clock, the instrument can be used to determine the time of year. The meridional 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. A fixed meridional ring on its own can be used as an
analemma calendar sundial, which can be read only at noon. 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 placed vertically, then rotated (relative to the celestial object) until it is parallel to the local north-south line. 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 supported by the meridional ring, it is often the outermost ring, as it is in the traveller's rings illustrated above. There, a sliding suspension shackle is attached to the top of the meridional ring, from which the whole device can be suspended. The meridional ring is marked in degrees of latitude (0–90, for each hemisphere). When properly used, the pointer on the support points to the latitude of the instrument's location. This tilts the equatorial ring so that it lies at the same angle to the vertical as the local equator.
Equatorial ring The
equatorial ring occupies a plane parallel to the celestial equator, at right angles to the meridian. It is aligned by • being attached to the meridional ring at the marking for latitude zero (see above) • being aligned to the declension ring, which is aligned to the celestial object. 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 on the declination ring (folded closed). 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 (see image). 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. ==References==