An astrolabe is a highly sophisticated scientific instrument used primarily for astronomical and astrological observations and calculations. An astrolabe is a two-dimensional version of a celestial globe that consists of a few elements that neatly come together to make a highly portable and versatile device. 
An astrolabe, usually circular, consists of some fixed and movable parts, of which inscribed discs are an important feature. The fundamental principle applied is the process of stereographic projection – in which a three-dimensional sphere is mapped-onto a two-dimensional plane. In this case, celestial spheres and related circles are projected on the crafted discs of the astrolabe. Of these discs the tympans or plates represent the sky as seen from the observer’s latitude, and contains engravings of the equator, tropics and other coordinates of the celestial sphere. A tympan is made specific to particular latitude; thus, an astrolabe usually consists of a set of tympans - for various terrestrial localities – usually major towns and cities.
Another crucial element is the rete which is another disc placed over the tympan. The rete, usually beautifully and intricately made, indicates the stars and constellations. This meticulously made, lace-like plate consists of many gaps and spaces that allow the lower plates to be legible. Thus, the freely moving rete over the tympan, simulates celestial motion over the select locality or latitude on any given day.