Western AstrolabesThe history of the astrolabe spans many centuries and cultures. Known in some form to the ancient Greeks, astrolabes were still being produced as astronomical instruments in the 19th Century. Over this period, many texts have been written about their construction and use. However, historians are still uncertain about the extent to which astrolabes were used for practical astronomy.
The instrument works on a principle called stereographic projection. This allows the 3-dimensional celestial sphere representing the heavens to be drawn on a flat disc marked with a grid of curved lines. The movements of the sun and stars can be traced against this grid, once their positions have been determined. Most astrolabes are equipped with sights to make the necessary observations, enabling the user to find the time of day or night. These sights can also be used to find the heights of buildings, trees and hills. A further function of the astrolabe was to model the appearance of the heavens for times past and future, making it useful in astrology.
Although some astrolabes were made of paper and wood, few of these have survived, and it is more common to see brass and gilt-brass examples. Many of these would have been very expensive, and may have been appreciated for their decorative value rather than as working instruments.
Knowledge of the astrolabe was conveyed to the Latin West through Islamic Spain in the Middle Ages. The oldest European instruments date from the thirteenth century (although there is disagreement over the dating of one particular instrument which may be older). Some of the most decorative astrolabes were made during the Renaissance, as commissions for royalty and nobility, but these may never have been used for astronomy. In the seventeenth century the astrolabe gradually fell out of use as its combination of observing, predicting and time-finding functions came to be carried out by more specialised instruments.
Astrolabes may have been most popular in the Latin West with medical doctors, since astrology played an important role in diagnosing illness and determining treatment. The ecliptic, which is the apparent annual path of the Sun around the Earth, is a feature on all astrolabe retes. With the aid of astronomical tables, it can be used to locate the planets as they move through the Zodiac - a band extending six degrees on each side of the ecliptic. This made the astrolabe a useful aide in the casting of horoscopes. Those astrolabes which lack the star pointers of other instruments or carry astrological information were probably intended for astrological use.