At the time of the rediscovery of the Azores navigation was, with the exception of the compass, without any of the instruments that later came into use. Prince Henry, however, realizing the importance of compiling information useful to mariners, systematized all the information available and erected an observatory to determine more accurately the data in reference to the declination of the sun. Most navigators use the sun far more than any of the other celestial bodies in order to determine their positions, and the first thing necessary is to know its declination—that is, its distance north or south of the equator. During the course of a year the movement of the earth, with its axis inclined at an angle to the plane in which it moves about the sun, brings the sun vertically over every section of the earth from twenty-three and one half degrees north of the equator to twenty-three and one half degrees south and back again. During the year, then, the sun is twice directly over our equator. Suppose at noon on one of these days a mariner wishes to determine his latitude, that is, his distance in degrees, minutes, and seconds north or south of the equator. He measures, with his sextant, the angle between the sun and the horizon. If he were on the equator that angle would be ninety degrees, for the sun would be directly over his head. He would then subtract the angle shown by his sextant from ninety, the number of degrees between the horizon and the zenith. In this case the answer would be zero. Therefore his latitude would be 0 degrees, and that is on the equator. If he were at the North Pole or the South, the sun would be on the horizon, and his sextant would show an angle of 0 degrees. Subtracting this from ninety he would find his latitude to be ninety degrees, north or south of the equator, as the case might be. At any position between the equator and the poles the problem would be worked in the same manner. But, except for two days in the year—but for two moments I might almost say—the sun is never directly over the equator, and declination is its distance at any given time north or south of the equator, measured in degrees, minutes, and seconds. This cannot be learned by any observations from a ship at sea. It is comparatively simple, however, to learn it by careful studies made at well-equipped observatories, and the results of these studies are now furnished mariners in carefully compiled form, so that they have merely to turn to their book in order to learn what the sun’s declination is at any given time. It was this work that Prince Henry began, and modern navigation may, perhaps, be said to have begun with his studies.