Even points ashore, where observations could be worked out under the best possible conditions, were thought to be from a few minutes to several degrees from what we now know are their positions, and when one realizes that an error of one minute of latitude means an error of one mile, it will be seen that an error of fifteen or twenty minutes might be enough to put a ship in grave danger while her captain thought her safe, and that a position in which there is an error of several degrees is little more than worthless, for each degree of latitude represents 60 miles, and three or four degrees mean one hundred and eighty or two hundred and forty miles. When it is realized, furthermore, that such errors as these were made ashore, where the observations were much more accurate than they could be at sea, one understands why seamen trusted their navigation but little, for they were often faced, no doubt, with errors of three or four hundred miles. And, if anything, their methods of determining latitude were less inaccurate than those used in determining longitude. Truly, navigation in those days left much to be desired.
Other instruments were invented from time to time in the struggle to master navigation. The “astronomical ring” was one, but it was little less crude than the astrolabe.
A SEXTANT IN USE
Sextants are used to measure the elevation of celestial bodies—the sun, moon, or stars—in working problems in latitude and longitude.
A SHIP’S LOG
The mechanism at the top is fastened on the ship’s rail, and a line with the rotator shown below at its end is allowed to trail in the water astern. The passage of the rotator through the water causes it to turn, the line is twisted, and the log is made to register the miles travelled.
Now up to the 16th Century navigators were without the one essential instrument necessary to the accurate determination of longitude. That instrument was an accurate timepiece that could be carried to sea. It is not necessary to have a timepiece in order to learn one’s latitude, but longitude is a more difficult problem, and time is an element in it. But the watches of the 16th Century were too inaccurate to be of much service, and, as a matter of fact, it was not until 1607 that it was realized that a day is not necessarily made up of twenty-four hours. If one stays in one place it is true that there are twenty-four complete hours from noon to noon, and clocks were designed to register the time at one place. But suppose, as the sun rises to-morrow morning, you board a very fast airplane and fly it at its fastest speed toward the west. Suppose this airplane flies at the rate of 1,000 miles an hour. In twenty-four hours you have flown around the world, and wherever you have been during that time the sun has been just rising behind you. It has been early morning for you all the time. Suppose, on the other hand, you had flown east at the same rate of speed. If you started at six o’clock in the morning, in three hours the sun would be overhead—that is, it would be noon for you. In three more it would be evening. In six more it would be morning again, for you would be halfway around the world. Six hours later evening would come to you, and in another six hours you would be at your starting point and it would be morning once more—the second morning you had seen after you started, but only the first morning after for the people you had parted from twenty-four hours before.
Ships, of course, do not travel at 1,000 miles an hour. But they do travel many miles, perhaps several hundred, in twenty-four hours. Therefore, if you start at Guayaquil, Ecuador (I use that, for it is very nearly on the equator), and sail west for twenty-four hours, making 240 miles, your watch will tell you that it is exactly the same time of day that it was when you left Guayaquil. But that is not true. It is the same time of day at Guayaquil, but you are four degrees west of Guayaquil, and the sun must still travel past four degrees of longitude before the time at the spot you have reached will be what your watch suggests. It will take the sun sixteen minutes to cover that distance, and therefore your watch is sixteen minutes fast.