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Astrolabes
Introduction
The 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 nineteenth
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.
Johann Stoeffler, Elucidatio fabricae
(Oppenheim: Jacob Koebel, 1524), 70
ususque
astrolabii
Using the shadow square on an astrolabe to measure the height of a
tower.
Reproduced by kind permission of the Syndics of Cambridge
University Library.
The instrument works on a principle called stereographic projection.
This allows the three 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.
To find out how an astrolabe was used in order to tell the time,
see this animation by the Museum of the History of Science,
Oxford, and the Science Museum, London.
Although some astrolabes were made of paper and wood, few of
these have survived, and it is more common to see brass and giltbrass 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.
Astrology
A further function of the astrolabe was to model the appearance of
the heavens for times past and future, making it useful in astrology.
Astrolabes may therefore have been most popular in the Latin West
with medical doctors, since astrology played an important role in
diagnosing illness and determining treatment.
Casting a horoscope requires a 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
apparent annual path of the Sun as seen from Earth is called the
ecliptic and the astrolabe can be used to calculate the position of the
Sun on this path. The ecliptic is divided into twelve equal sections,
representing the signs of the zodiac; the position of the Sun on the
ecliptic determines which zodiac sign it is in.
The ecliptic is a feature on all astrolabe retes. With the aid of
astronomical tables, an astrolabe enabled non-mathematicians to
calculate positions and angles of the heavenly bodies. Those
astrolabes which lack the star pointers of other instruments or carry
astrological information were probably intended for astrological use.
Jacob Koebel, Astrolabii Declaratio Eiusdemque Usus (Mainz:
Peter Jordan, 1535), sig. D1r
Exemplum for finding the beginnings of the twelve houses with an
astrolabe for a given time.
Reproduced by kind permission of the Syndics of Cambridge
University Library.
Hipparchus and Ptolemy on Astrolabes
Hipparchus (fl. the second half of the second century BC) was a
careful astronomical observer, who operated in a systematic fashion.
He may have been responsible for the invention (or adaptation) of
several astronomical instruments. Some historians of astronomy
(including G. J. Toomer, John North and David King) believe that
Hipparchus may have been the inventor of the planispheric
astrolabe, used to tell the time at night from stellar positions. The
earliest surviving description of the planispheric astrolabe is to be
found in the writings of John Philoponus, who lived during the sixth
century AD, long after Hipparchus. However, the mathematical
theory which serves as the foundation for the stereographic
projection used in the planispheric astrolabe was provided in the
second century AD, by Ptolemy, in his Planispherium. Given
Ptolemy's acknowledged debt to his predecessor, it is entirely
possible that he was here, too, building on the work of Hipparchus.
Gemma Frisius, De Astrolabo Catholico libellus (Antwerp: ex
officina Arnoldi Coninx, 1584), p. 358
A visual account of projection.
Reproduced by kind permission of the Syndics of Cambridge
University Library.
Ptolemy's Planisphaerium (as it was known in the Middle Ages)
dealt with the problem of mapping figures from the celestial sphere
onto a plane by a specific method that preserves circles, which is
now known as 'stereographic projection'. The early interest in this
technique, which is documented well before Ptolemy, may have
been a desire to simplify certain problems in spherical geometry by
reducing them to plane geometry. Stereographic projection later
became the mathematical basis of the plane astrolabe. (It should be
noted that the 'armillary astrolabe' or star-taker made of rings or
bracelets described by Ptolemy in the Almagest Book 5, chapter 1,
was a completely different sort of instrument).
Johann Stoeffler, Elucidatio fabricae ususque astrolabii
(Oppenheim: Jacob Koebel, 1524), 6
Marking out a latitude plate for an astrolabe.
Reproduced by kind permission of the Syndics of Cambridge University
Library.
Regiomontanus and the Astrolabe
During his years in Vienna, Johannes Regiomontanus (d. 1476)
composed a tract on the construction and use of the astrolabe.
Documentary evidence suggests that he later designed several of
these instruments, including one presented to King Matthias of
Hungary which is now lost. An extant astrolabe of 1462, signed
‘Johannes’ and dedicated to Cardinal Bessarion, is probably by
Regiomontanus.
At least eleven other ‘Regiomontanus-type’
German astrolabes of the fifteenth century, with significant
similarities to the Bessarion instrument, survive, indicating a distinct
tradition which Regiomontanus established or participated in. The
retia of these instruments are based on an earlier Italian form, and
they feature Roman lettering as well as Gothic script. Notably,
Regiomontanus introduced the Roman font to Nuremberg when he
established his printing press there. It is apparent that elements of
the design of these astrolabes were adopted by Johannes Stöffler
(1452-1531), the first Professor of Mathematics and Astronomy at
Tübingen University, and Georg Hartmann (1489-1564), the leading
Nuremberg instrument-maker of the early-sixteenth century.
The Bessarion astrolabe, and several others in the same tradition,
have on the reverse an organum Ptolemei: a projection valid at any
latitude for determining the time in equinoctial hours from the
altitude of the Sun for any latitude. This projection, with the
addition of stellar positions, is the basis of the form of universal
astrolabe commonly (but for this reason misleadingly) called the
Rojas-type, after a publication by Juan de Rojas which described it
in 1550. Regiomontanus himself is known to studied this theory in
the treatises of earlier Viennese mathematicians.
Further reading
M. Hoskin (ed.) The Cambridge Illustrated History of Astronomy
(Cambridge, 1997)
D. King, Astronomy in the Service of Islam (Aldershot, 1993)
G. L'E. Turner and D. King, ‘The Astrolabe Presented by
Regiomontanus to Cardinal Bessarion in 1462’, Nuncius 9 (1994),
pp. 165-206
E. Zinner, Leben und Wirken des Joh. Müller von Königsberg
(Osnabrück, 1968). Translated by E. Brown as Regiomontanus: His
Life and Work (Amsterdam, 1990)
D. A. King, 'Astrolabe' in Instruments of Science: An Historical
Encyclopedia (London, 1998)
J. North, The Fontana History of Astronomy and Cosmology
(London, 1994)
S. Schechener Genuth, ‘Introduction’ to R. S. Webster and M.
Webster (eds.) Western Astrolabes (Chicago, 1998)
G. J. Toomer, 'Astronomical Instruments' in Oxford Classical
Dictionary, 3rd ed. (Oxford, 1996)
G. J. Toomer, ‘Hipparchus’ in Oxford Classical Dictionary, 3rd ed.
(Oxford, 1996)
Text by Graham Hart, Adam Mosley and Liba Taub 2009
G. J. Toomer ‘Ptolemy’ in The Dictionary of Scientific Biography
(New York, 1970), pp. 186-206
http://www.hps.cam.ac.uk/starry and
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