Ships of the seven seas

by Hawthorne Daniel

Columbus, of course, did not begin the era of discovery. Prince Henry, the “navigator,” sent out an expedition from Portugal in 1432 which rediscovered the Azores—an astonishing thing for times so early, for the Azores lie 830 miles west of Portugal and are farther from a continental mainland than any other of the islands of the Atlantic. That the islands were known to the ancients, however, is proved by numerous Carthaginian coins found on the island of Corvo, but their location and practically everything else concerning them seems to have been lost until Henry the Navigator attached them to Portugal.

But the rediscovery of the Azores proves only that the sailors put great faith in their compasses, and sailed, despite their fears, out to the west where all of them knew (it was no matter of mere belief) that the sea ended somewhere suddenly, and that their cockleshell ships would, if they but sailed to the edge, fall down the smooth green cataract of an awful, endless waterfall, into limitless space, or, mayhap, to the vast foundations upon which the world was built. To them it was as if a canoe were being paddled downstream to the brink of a cataract to which Niagara itself would be but a raindrop falling from the eaves.

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.

But all the tables of declination are of no use without an instrument with which to measure accurately the angle between the sun and the horizon, and such an instrument was slow in coming. The first apparatus used was called a “cross staff.” It was made of two rods, one about thirty-six inches and the other about twenty-six inches long. The shorter staff was arranged so that its centre slid along the other while it remained rigidly at right angles to the longer staff. To work out one’s latitude with this instrument the observer waited until noon was almost upon him. He then took his cross staff and, placing one end of the longer crossbar to his eye and holding the instrument so that the shorter bar stood in a vertical plane, moved the shorter bar back and forth until he could sight over the upper end at the sun and, at the same time, beneath the lower end at the horizon. As the sun continued to mount to its highest point he pulled the cross staff slowly toward him, measuring a greater and a greater angle. When the sun had reached its highest point and the angle between it and the horizon began to lessen, his “sight” was completed, and carefully holding the crossbar where it marked the greatest angle he laid it on a table on which a circle was inscribed. The end that had been at his eye he placed at the centre of the circle, and the segment marked by the lines from the centre past the two ends of the crossbar showed the number of degrees in the angle he had measured between the horizon and the sun.

But any one who has attempted to sight a gun accurately while standing on an irregularly moving platform will have some idea of the difficulty these old sailors had in sighting accurately at the horizon and the sun at identically the same time from the deck of a bobbing ship. The glare of the sun, the motion of the ship, and the inaccuracy of the instrument itself could not be expected to give more than approximate results, especially as the several corrections on the angle now known to be necessary (the refraction of the sun’s rays as they enter our atmosphere is one) were either not recognized or were inaccurately known.