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PART 3 – ASTROLABES
Introduction to astrolabes
Note: please refer to the glossary to find the definitions of technical terms used in this text.
An astrolabe is a representation of the celestial sphere on a plane. It is an astronomical calculator used for
example to find hours of setting or rising of the Sun or of a star, or to convert coordinates of a star from a
coordinate system to another (horizontal, equatorial, ecliptic), or to find azimuth, altitude, right ascension,
declination, etc.
Already used by Greeks, astrolabes were specially developed by Muslim astronomers around the 15th and
16th centuries.
Astrolabes usually have drawings on their two faces (front and the back) whose purpose may be different
and/or complementary.
The front of the astrolabe
The front of an astrolabe is constituted with the mater (2) on which several mobile disks and accessories are
installed. The mater can be suspended by a ring called the throne (not drawn here). The limb (1) is around the
mater and is thicker. The mater, throne and limb are fixed with respect to each other. The mater has a hole in
its center. Plates can be installed on the mater that are designed for a given latitude. An astrolabe is usually
provided with at least three plates in addition to the mater which can contain also the same drawings. The
Rete (3) is installed on top of the plate, then sometimes a Ruler (6).
1. The Limb – is around the mater and scaled in
degrees (90° quadrants) and in hour angles. It
shows north, south, east and west points and is
usually used with the ruler.
2. The Mater – can have the same drawings as a
plate. The three circles are the two tropics and the
equator. The drawings inside the horizon are
azimuth and altitude arcs (almucantarats).
3. The Rete – can be rotated around the central
axis. It is usually decorated with fine and rich
details.
4. The Ecliptic circle – is scaled in ecliptic longitude
and displays Zodiac symbols. It is part of the Rete.
5. Stars of the celestial sphere – can be drawn
directly or may be replaced by small pointers
attached to the Rete.
6. The Ruler – rotates around the center and points
to a scale on the limb.
User manual of Shadows Pro version 4.0 – © François Blateyron – page 63