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.