the moon, whilst it lengthens its distance from the ecliptic, has more degrees towards the west than towards the east, it must necessarily have more latitude on one side (of the globe) than on the other: and when the latitude is known, by measuring the degrees and minutes with the astrolabe, it will be known whether it is found, and how far it is found towards the east or the west. But in order to ascertain the longitude, you must know in what latitude the moon ought to be at that same moment in the place from which you sailed, for instance, in Seville. By knowing the latitude and longitude of the moon at Seville in degrees and minutes, and seeing also the latitude and longitude which it has in the place where you are, you will know how many hours and minutes you are distant from Seville; and afterwards you will calculate the distance in east or west longitude.
"ii. The moon furnishes another method for ascertaining the longitude, but that is when I knew the precise hour in which the moon observed at Seville ought to be in conjunction with a given star or planet, or ought to be in a certain opposition to the sun, of which the degrees are determined: and this I can know by means of an almanack. And since that happens in the east before it happens in the west, as many as may be the hours and minutes that may elapse from the time when the conjunction took place at Seville, till the time in which I observe it to take place, so much will be my longitude west of Seville. But if I should see the conjunction take place before the hour in which it ought to happen with respect to Seville, then my distance in longitude will be east. For each hour, fifteen degrees of longitude are calculated.
"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
