At first glance, the world map by Fra Mauro (active ca. 1430-ca. 1459/1464) seems to represent half of the terrestrial globe; in geographic terms, a hemisphere of 180 degrees. The center of the world, however, corresponds not to the equator but to the latitude of Jerusalem, which is found 31 degrees north of the equator. It is also evident that the northern territories of the globe extend all the way to the North Pole, situated in proximity to the circumference, and thus the radius of the circle necessarily measures 60 or 65 degrees.
Such is the case also with the longitudinal distance between the central meridian—passing through the Caspian Sea—and the western coast of Africa. According to these data, the entire world map would thus represent a spherical cap of 130 degrees, as seen in the cartouche dedicated to the cosmography of the elements. This would lead to locating the extreme southern extension of Africa to approximately 35 degrees south of the equator, which is more or less where we actually find the Cape of Good Hope.
Given that the central meridian of the world map corresponds to the one chosen by Ptolemy(ca. 100-ca. 178), passing through the Caspian Sea and the Persian Gulf, and because for the Alexandrine geographer the longitudinal extension of the known world came to 180 degrees, Fra Mauro seems to have re-dimensioned the longitudinal distances so as to include, within the 130 degrees he calculated, also the territories explored by Marco Polo (1254-ca. 1324) beyond the oikumene (i.e. the inhabited world) recognized by the ancients.
While to the east of the central meridian the extension of lands was entirely conjectural, to the west Fra Mauro could count on the great tradition of seafaring navigation, which over the course of the Middle Ages had produced ever more exact nautical charts of the Mediterranean, and which revealed the excessive longitudinal extension of Ptolemy’s planisphere.
The marine chart which Fra Mauro himself composed, now in the Vatican Library, can be perfectly superimposed onto the western half of his world map, to such a degree that detailed calculations can be made concerning the size of the habitable world.
Setting aside the counter-clockwise rotation due to magnetic declination (a typical feature of nautical charts), his profile of the Mediterranean corresponds almost perfectly to geographic reality.
Having established for the Mediterranean a longitudinal extension of 42 degrees, the scale of miles found on the short edge of the marine chart tells us that—at that latitude—a parallel arc of 10 degrees measures circa 690 nautical miles. By reproducing this measure on the same arc on the equator, the entire circumference of the Earth would measure 31,000 miles, circa 37,000 kilometers, a value analogous to the one in the cartouche dedicated to the distance between the heavens and longer than Ptolemy’s estimates. In comparison with Ptolemy, the size of the habitable world for Fra Mauro was less extended in longitude and far more extended in latitude, and was perfectly inscribed within the circular shape of the world map.