The annulus astronomicus (known
as the astronomical rings) seems a simple device, but it plays
a prominent role in Cosmographia. It serves many of the
same functions as other instruments in Cosmographia such
as telling the time. However, this set of three fitted divided
degree circles is ingenious in that it is self-orienting. Unlike
most time telling instruments (that need to a compass to help
find the North-South meridian), this instrument itself identifies
the meridian when the shadows of its rings come into conjunction.
The annulus astronomicus was designed by Gemma Frisius (although
its lineage dates to Ptolemaic times), and is also called 'Gemma's
Rings'. Cosmographia provided a debut for the rings and
its maker with a convienient piece of advertising.