"The ten circles of the armillary sphere, of which the six major pass through the center of the earth, serve to determine the situation of countries and climates. The Ecliptic determines the movement of the sun and the planets: the two Tropics indicate the point to which the sun declines from the equator towards the North in summer, and towards the South in winter. The Meridian, always variable, because it passes through all points of the equator, cutting it perpendicularly, designates the longitude, and it is on it that the latitudes are marked."
Of Latitude.
After having well explained the armillary sphere with all its parts, and their use according to the system of Ptolemy, the author goes on to teach the method of taking the altitude of the pole, on which the latitude is calculated; fixing the pole at 0° and the equator at 90°.
"The Polar star," he says, "is not precisely on the point corresponding to the axis of the earth; but it turns round it, as do all the other stars. In order to know its true position with regard to the pole, it must be observed where the Guard stars[1] stand. If these are on the western arm,[2] the polar star stands one degree above the pole: if they
- ↑ The guard stars are 6 and 7 of Ursa Minor, which form a triangle with the pole and pole star; now 7 at the belt of Cassiopeia is used. Note, Milan edition.
- ↑ This means the arm of the instrument used; it might be the meteoroscope of Regiomontano, which had a cross in the middle: or an astrolabe like it; or the common astrolabe with a dioptron, or mediclino, as Pigafetta calls it, placed on the equator. Note, Milan edition.
half land and half sea, and then giving to each league three and a half miles, we should have 22,050 miles for the circumference of the earth: a measure very little differing from that which results from giving to each degree at the equator sixty Italian miles, by which the circumference is 21,600 miles. Note, Milan edition.
