Astrolabes

An astrolabe is a plane representation of the celestial sphere obtained by a special projection called sterographic projection. But it shall be considered more as an astronomical calculator allowing to calculate rising, setting and meridian crossing hours of any astronomical body (star, planet, Sun), to convert coordinates from one system to another (horizontal, equatorial, ecliptic), or to find azimuth, altitude, right ascension, or declination of a star, etc.

Use during ancient times by Greeks, the astrolabe was really developed and improved by muslim scientists around the 15th and 16th centuries.

The astrolabe has a net of lines on both sides, for a different and complementary usage. Therefore, front and back of an astrolabe are described separately.

Differents types of astrolabes

Planispheric astrolabe	Universal astrolabe	Rojas astrolabe
Islamic astrolabe	Mariner's astrolabe	Astrolabe quadrant

The back of an astrolabe

1 - the limb of the back is scaled with various information. A calendar gives days and months and allows the determination of the ecliptic longitude in function of the day. The 1/4 of circles are scaled in degrees for using with the alidade.

2 - the alidade is rotated around the central axis and is used to point a star or the Sun with the two pinholes to get its altitude. It is also used to designate a value on the limb.

3 - shadow square.

4 - curve of equation of time, used with the alidade's scale and a date on the limb.

5 - unequal hours.

Photos:
- top banner: detail of an ancient astrolabe.

Astrolabe - Astrolabio - Astrolábio - Astrolabium - asztrolábium - aстролябия