There were other writers who were more or less sceptical. Washington Irving rejected the story. Crantz and Graah, eminent Danish travellers and writers, were doubtful, and more or less incredulous. Admiral Zarhtmann of Copenhagen rejected both narrative and map, as did the learned Danish writer Steenstrup.
All this was before the discovery of medieval maps which exposed the whole imposition. These were, especially, the large map of Olaus Magnus (Venice 1539), found in the Munich library in 1886, and the Zamoiski map (1467), discovered at Warsaw in 1888; also a map of North Europe and Greenland in the MS. Ptolemy at Florence, and the edition of Ptolemy published at Ulm in 1482—the earliest printed map showing Greenland.
Most of the names on the Zeno map were supposed to be original; due to their discoveries, and not existent on any earlier map. The discovery of these earlier medieval maps, however, has disposed of that delusion. Of the 19 Zeno names on Iceland, 12 are in the Zamoiski map, 3 in the Florence map, and the others in that of Olaus Magnus. On the Cantino map in 1502 appears Frisland, placed due north of Scotland. It is a clerical error in copying Stillanda from the Cosa map. This is the way Zeno got hold of the name Frislanda. The whole was concocted by Niccolo Zeno and his publisher Marcoloni in 1558, from materials on maps then existing.
The Zeno imposture was first studied by Professor Storm, in the light of the Zamoiski and Olaus Magnus maps, and he exposed the falsities of the narrative, and the imposture of the map. The whole subject was discussed in an exhaustive work by Mr F. W. Lucas, from which the above details have been taken[26]. The mischief done by the Zeno forgery, while it lasted, was very serious; causing confusion in the work of cartographers as well as mistakes in the reports of navigators.
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In the period of the beginning of English Arctic exploration, the instrument mainly used for finding the latitude was the astrolabe. The cross-staff had been invented, but was not in general use, nor was the quadrant with a plumb-line, though it had been used by Columbus. The astrolabe was a circular metal ring with inlet plates and discs. These plates were fitted to drop into an inner depression of the ring, the principal one being called the rete. It consisted of a circular plate marked with zodiacs sub-divided into degrees, with narrow branching limbs having smaller tongues terminating in points, each denoting the position of a star. The plates, or “tables” as Chaucer calls them, were differently marked for places having different latitudes. Within all these scales of Umbra recta and Umbra versa there is a division into 12 parts for taking and computing heights and distances by an approximate method. The alidada is a straight-edge across the ring moveable with two sights, and a pin ties them all together.
Astrolabe in Gonville and Caius College, Cambridge (early 14th century)
The alidada is for taking the altitude of the sun, and the rete adjusted to this altitude shows the hour of the day. To take an observation the right thumb is put into the ring of the astrolabe, and the left side is turned against the light of the sun. The alidada or rule is moved up and down until the rays of the sun shine through both sights. Then the number of degrees the alidada is raised from the little cross placed to show the east line is the altitude of the sun, read off on the outer ring. The Spaniards constructed their astrolabes small and heavy, to prevent them from being blown about, not much over five inches in diameter yet weighing 4 lbs. The diameter of the English astrolabes was six or seven inches, sometimes more.
This instrument, invented by Hipparchus and developed by Ptolemy, was in use until the days of Elizabeth. It has a peculiar interest for those who are fond of studying the history of maritime discovery, but it is by no means simple in construction and it is necessary to examine the astrolabe itself to understand it and its uses[27].