"The true meridian may also be ascertained, or rather the equinoctial line, which cuts the meridian at right angles, by observing the points where the sun rises and sets, and by observing how much they are distant from the equinoctial either to the north or to the south. For this purpose an astrolabe is formed with the globe; that is, a circle is made representing the earth's circumference, divided into 360 deg. At sunrise fix two pins in the circumference, in such a manner that a line drawn from one to the other should pass through the centre, and place the pins so that both should be in a line opposite the sun's center. Place two other pins in the same way in the circumference when the sun sets. You will thus see how much the sun declines from the equinoctial line, either to the north or to the south. And as many degrees as the pins are distant from the equinoctial, so many degrees the the sun's declination. Having found the sun's rising and setting, you will also find the medium distance; that is, the meridian line, and afterwards you will see how much the compass or magnetic needle north-easts or north-wests. You will infer from this how far you are from the Fortunate islands; that is, from Tenerife towards the east or the west. This method has been tried by experience.[1]
- ↑ Amoretti, in his introduction to this Treatise of Navigation, in the Milan edition, observes that Pigafetta was misled by a false theory when he supposes that there is in the heavens a point in repose to which the magnetic needle tends, but that the exact direction of the magnetic needle coincided, or at least approximated to the meridian of the isle of Ferro, which is not now the case; and that in some other places the variation of the compass had been observed to correspond with that of the longitude. By the table of variations of the compass published by Lambert in the Ephemerides of Berlin (Astronomische Jahrbuch) for the year 1779, it is seen by an easy calculation that at the beginning of the sixteenth century the magnetic equator or zero of deviation was very near the isle of Tenerife. Now it is further off, and the distance
round it, and is named sometimes albidade, or dioptron, or traguado, or linea di fiducia. Note, Milan edition.
