“If it is not possible to take the sun’s altitude at midday, that can be determined with an hour-glass of sand, taking the hours of the night from sunset till the moment of its rising. Having learned the hours of the night, you will know how many are wanting of the twenty-four, and these you will divide into two equal parts. When half of this has elapsed, be sure that it is midday, and that the shadow indicates to you the true meridian. But since the sand clock may often be inexact, it will be better to take the sun’s altitude with the astrolabe by means of its mediclino.[431]” “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 are 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.[432]” +Direction of the Ship.+ “If you wish to navigate to any place, you must first know its position; that is, its latitude and longitude. Then, by means of the compass you will point directly to that place. And since the compass varies to east and west, you must, by the methods above described, ascertain its variation, and subtract or add that which is necessary, so that the ship’s head, regulated by the compass, may have the required direction.”