Freiburger historische Bestände - digital

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I must leave it to the sound judgment of my readers
in how far extravagant claims, such as these, can be
reconciled with what we know of Behaim and of the
history of Portugal in the time of John II.

The Astrolabe.

Far less fanciful are those authors who merely credit
the Junta with having " invented" the astrolabe or
" adapted " it to the use of navigators. One of the earliest
among the former is Manuel Pimentel ('Arte de Navigar,'
Lisbon, 1682), whose lead was followed by Admiral
Ignacio da Costa Quintella ('Annaes da Marinha
Portugueza,' Lisbon, 1839, I., 190). Thus the Junta
might merely have simplified the planispheric astrolabe of
the astrologers, so as to adapt it to the use of seamen.
This planispheric astrolabe, of which numerous examples
may be seen at the British Museum,1 had a shallow well
—the mater astrolabii—within which was placed an
engraved chart of the heavens as seen in a given latitude,
on a stereographic projection. An ingeniously devised
Reta or Arenea (Spider's Web) moved concentrically

The Astrola.be of Regiomontanus, 1468.

above this chart, and by means of it certain astronomical
problems could be solved graphically. The improvement
by Regiomontanus merely consisted in a device
which rendered the instrument more useful to astrologers
desirous of finding the " initia coelestium domi-
ciliorum," a device of no interest whatever to the
seaman.2

1 One of its earliest descriptions is by Hermann Contractus of
Vehringen, a pupil of the Convent School of Reichenau, who wrote ' De
mensura astrolabii.' The author died in 1054.

2 Breusing, ' Die nautischen Instrumente,' Bremen, 1890, p. 3.

The seaman's astrolabe,3 as shown in the illustration,
was a much more simple instrument. A disc or a ring of
wood or metal, 3 to 15 inches in diameter, was crossed by
fines representing the horizon and zenith. The rim
between the horizon and zenith was divided into 90
degrees. A movable rule or alidade, with sights, turned

The Seaman's Astrolabe.

round a point or pin in the centre of the circle. The
observer sat with his back to the main mast, held the
astrolabe in suspension on a finger of his left hand, whilst
he moved the rule up and down with his right until the
sun was on with both sights. Supposing the astrolabe
had been suspended vertically, this yielded a correct,
though somewhat rough, altitude of the sun. Of course,
on deck a rolling vessel, the results obtained even by a
skilled observer were frequently far from satisfactory, and
Master John, one of the pilots of Cabral's fleet, tells us
that errors of four or five degrees in the resulting latitude
were almost unavoidable.4 Hence, when it was desired to
obtain trustworthy results, the observer landed and set up
an astrolabe of larger dimensions upon a tripod stand.
This was done by Vasco da Gama when he arrived at the
Bay of St. Helena.

Murr5 was the first to connect Behaim with
the introduction or adaptation of the astrolabe for
the service of the Portuguese marine, and his view
was accepted by Francisco de Borja Garcao-Stockler,6

8 Abulwefa, a famous Arab astronomer of the ninth century, clearly
describes this simple instrument (L. A. Sedillot,' Mem. sur les instruments
astronomiques des Arabes,' Paris, 1841, p. 195).

4 ' Alguns documentoB,' p. 122.

* 'Dipl. Goschichte Martin Behaims' (Gotha, 1778); 2nd edition,
1801, p. 72.

6 ' Ensaio hist, sobre a origem e progresso das mathematicas em
Portugal" (Paris, 1819).