The astrolabe is a device for solving problems relating to time and the position of the Sun and stars in the sky. Several types of astrolabes have been made. By far the most popular type is the planispheric astrolabe, on which the celestial sphere is projected onto the plane of the equator. The name astrolabe is derived by aster and lambanein, i.e., star and receive or get and means combined a device that finds a star. The word stereographic is derived from stereos “solid” and “graphein” to write, i.e. it describes the drawing of the projections of solid objects. A stereographic projection is used for the mapping of the celestial sphere on the astrolabe plane (a so called conformal -angle preserving – projection, that maps circles to circles).
Astrolabes are used to show how the sky looks at a specific place at a given time. This is done by drawing the sky on the face of the astrolabe and marking it so positions in the sky are easy to find. To use an astrolabe, you adjust the movable components to a specific date and time. Once set, the entire sky, both visible and invisible, is represented on the face of the instrument. This allows a great many astronomical problems to be solved in a very visual way. Typical uses of the astrolabe include finding the time during the day or night, finding the time of a celestial event such as sunrise or sunset and as a handy reference of celestial positions. Astrolabes were also one of the basic astronomy education tools in the late Middle Ages. Old instruments were also used for astrological purposes. The typical astrolabe was not a navigational instrument although an instrument called the mariner's astrolabe was widely used. The mariner's astrolabe is simply a ring marked in degrees for measuring celestial altitudes.
The origins of the astrolabe were in classical Greece.