The astrolabe is a portable astronomical calculating instrument. It can be used to perform all sorts of complex calculations using the position of the Sun or stars.
The mater (Latin for mother) is the main body of the astrolabe. The edge of the mater is called the limb, on which are the degree scale and scale of hours are engraved. The hollowed-out part of the mater is called the womb and contains the latitude plates.
To an observer on the Earth it appears that our planet is at the centre of an immense sphere with the stars and other heavenly bodies located on its inside surface. Called the celestial sphere, it appears to rotate around the Earth.
The celestial sphere is mapped on the plate of an astrolabe using a mathematical technique called stereographic projection. This technique allows the 3-dimensional sphere to be represented on the 2-dimensional flat plate. Each latitude needs its own projection, and so most astrolabes come with a variety of plates for particular latitudes. usually stacked one on top of the other, within the astrolabe.
This cut-out plate (sometimes called the star-net) carries the star pointers and ecliptic ring and can be rotated over the latitude plates underneath.
The rule is a bar which rotates across the front of the astrolabe and is used to locate positions on the plate or rete, and to relate them to the scale of hours marked on the limb.
The alidade is a rotating bar, found on the back of an astrolabe. Unlike the rule the alidade has little vanes with pin-holes or slots at each end which are used as sights. Altitude is measured by lining up an object, such as a star, in the two sighting holes, and then reading off the altitude in degrees on a scale around the edge.