“To understand this does not require any great genius. It should be borne in mind that the moon has a motion opposed to the general motion of the heavens; that is, it goes from west to east, and in every two hours it progresses a degree and a few minutes; and since it is in the first heaven, and the stars are in the eighth, it certainly never enters in conjunction with them; but sometimes it interposes itself before the rays which come from them to our eye: but this does not happen at the same time to those who are at Seville, and to those who are at Valencia. The annexed figure will give an idea of this, from which it is seen that the ray of the star +d+ is intercepted by the moon +c+ for those who are at +a+, and not for those who are at +B+, for whom it was intercepted when the moon was at +e+.” “III. The compass can also supply a method, still easier, for finding the longitude of the place in which you are. It is known that the compass, or the magnetised needle which is in it, directs itself to a given point, because of the tendency which the loadstone has towards the pole. The reason of this tendency is because the loadstone does not find in the heavens any other spot in repose except the pole, and on that account directs itself towards it. This is an explanation of the phenomenon which I propose; and I believe it to be true, so long as experience does not inform us of some better explanation.” “In order to know, by means of the needle, the degrees of longitude, form a large circle, in which place the compass, and divide it into 360 deg.: and having placed the needle at 360 deg., where it indicates the arctic pole; when the needle is in repose, draw a thread, which should pass from the arctic pole, pointed out by the needle to the antarctic pole, and let this thread be longer than the diameter. After that take the south, which you will know by the greatest altitude of the sun. Turn the compass, until the thread which traverses it coincides with the direction of the meridian shade; then, from the antarctic pole of the needle, with the thread which remained over, draw another thread to the arctic pole, that is, to the flower;[427] and you will thus find how many degrees the needle of the compass is distant from the meridian line, that is, from the true pole. So many will be the degrees of longitude, which you will have from the place where the compass begins to set itself in motion.[428] Therefore, with the more accuracy you take the true meridian so much the more exactly will you be able to ascertain the degrees of longitude. And from this it may be seen that the meridian should never be taken with the compass, because it north-easts or north-wests,[429] as soon as it goes out of the true meridian; but take an observation of the south[430] with the astrolabe, and judge that it is midday when the sun is at its greatest height.”