Astronomical rings

The instrument consists of three rings, representing the celestial equator, declination, and the meridian.

[3] 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 three rings are oriented with respect to the local meridian, the planet's equator, and a celestial object.

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.

[clarification needed] 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.

There, a sliding suspension shackle is attached to the top of the meridional ring, from which the whole device can be suspended.

This tilts the equatorial ring so that it lies at the same angle to the vertical as the local equator.

[5][6] The equatorial ring occupies a plane parallel to the celestial equator, at right angles to the meridian.