Casting
a horoscope requires knowledge of the relative positions of the sun, moon,
stars and planets in order to draw conclusions and make predictions. One of the
fundamental calculations in astrology is the position of the sun relative to
the signs of the zodiac. The path of the sun as seen from Earth is called the
ecliptic and the astrolabe calculates the position of the sun on this path. The
ecliptic is divided into 12 equal sections, representing the signs of the
zodiac, so the position of the sun on the ecliptic determines the zodiac sign
it is in. The use of an astrolabe simplified greatly this process, enabling
non-mathematicians to calculate positions and angles of the heavenly bodies.
Astrological
works were widely available, and astrological calculations were among those
considered most important in texts on the astrolabe. The treatise on the
astrolabe attributed to Messahalla explains 47 uses of the astrolabe, of which
about half are directly related to astrological calculations. Collections of
tables were often included with treatises on the astrolabe explaining how to
derive things like the length of someone's life from astronomical data obtained
with the astrolabe. Engraved on the back of some astrolabes are tables of
astrological information, such as the signs, their "lords" and
characteristics. Scales of purely astrological use are found on the back of some
very small astrolabes (on which space would have been at a premium), further
indicating the importance of astrology to Arabic users of the astrolabe.
Judicial
astrology (the prediction of future events from the heavens) was more
acceptable to Islam than to Christianity since it was not seen to threaten free
will or the authority of Allah in the way that astrology was seen to threaten
the sovereignty of the Christians' God in the medieval period. In the Qu'ran it
is written that God created the stars and that men should be guided by them,
indicating the more accepted place of astrology and astronomy within Islam.
Let’s Continue…
The
earliest surviving Arabic astrolabe treatises are from the seventh and eighth
centuries and are often translations of earlier Greek or Syriac texts. Eighth
century literary references from Baghdad and Damascus indicate that by this
time the use of the astrolabe was widespread throughout the Arab world. Land
under Arab control stretched from North Africa and Spain to India, enabling a
wide range of astronomical influences to be combined. The early ninth-century
tables of al-Farghânî list the radii of the circles on the plate of the
astrolabe for each degree of latitude. These simplified the process of
astrolabe construction by removing the need for mathematical calculation of
these values, indicating that astrolabes were being manufactured in substantial
numbers since the effort involved in producing the tables would have been
considerable. The earliest surviving Islamic astrolabes date from the ninth
century, and these are of such quality and craftsmanship that they represent a
continuing tradition rather than a new activity. By the eleventh and twelfth
centuries there are many surviving texts and astrolabes, the instruments
varying in style and artistry but retaining many fundamental similarities in
functionality and design.
This
astrolabe is signed "Husain b. Ali" and dated 1309/10 AD. It is
probably North African in origin, and is made of brass. It has four plates (for
the front of the astrolabe, representing the projection of the celestial sphere
and marked with lines for calculation), each for a specific latitude, and 21
stars marked on the rete (the star map, with pointers, fitting over the plate).
I have chosen this astrolabe since it is right in the middle of the time frame
for Islamic astrolabe use (ca. 600 to ca. 1800) and because it demonstrates
many of the features common to Islamic astrolabes.
On the
back is a shadow square for measuring the heights of inaccessible things and
other similar calculations (shadow squares are quite common, but not on all
astrolabes), and scales for calendrical calculations and calculation of the
qibla (the direction to face during prayers).
A typical
text on the astrolabe describes more than forty uses of the astrolabe,
indicating its versatility as an astronomical calculating device. Some of its
principal uses to the Islamic astronomers were to provide answers, to
astrological calendrical, and meteorological questions. Although less accurate
than direct mathematical calculations (the astrolabe is only as accurate as the
positioning of the rete and so on) it provided an easy and quick way to
calculate values.
The Arab
theory of winds centred on four cardinal winds and is recorded mainly in
folk-astronomical texts, suggesting that it was an old and established
tradition. The four cardinal wind directions were astronomically defined: the
qabûl was defined as being from the direction of the summer sunrise, and the
janûb as being in the direction of the rising of the star canopus. These four
cardinal winds are roughly perpendicular but are not equivalent to north,
south, east and west: they are offset by about 25°. David King has suggested that
the sacred Ka'ba in Mecca is oriented according to these four cardinal winds,
an association found in several medieval Islamic texts. In addition, Arabic
folklore associates the sides of the Ka'ba with winds and rain. Some astrolabes
include plates with markings related to this wind theory, indicating the
importance of the instrument as a way of calculating the directions of the
winds and possibly therefore calculating the orientation of the Ka'ba.
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