Page:Popular Science Monthly Volume 3.djvu/742

724 compiler would be tolerably sure to do, unless he had a mathematical friend at his elbow) that, by voyaging eastward at such and such a rate, a traveller can always have the moon "full" at night, in what an unpleasant predicament would the mistake have placed him! Such things happen, unfortunately; nay, I have even seen works, in which precisely such mistakes have been made, in use positively as text-books for examinations. On this account, our fiction writers must be careful in introducing science details, lest peradventure science-teachers (save the mark!) be led astray.

It need scarcely be said that no amount of eastwardly voyaging would cause the moon to remain always "full" as seen by the voyager. The moon's phase is the same from whatever part of the earth she may be seen, and she will become "new," that is, pass between the earth and the sun, no matter what voyages may be undertaken by the inhabitants of earth. Mr. Clemens has confounded the monthly motion of the moon with her daily motion. A traveller who could only go fast enough eastward might keep the moon always due south. To do this he would have to travel completely round the earth in a day and (roughly) about 50½ minutes. If he continued this for a whole month, the moon would never leave the southern heavens; but she would not continue "full." In fact, we see that the hour of the day (local time) would be continually changing—since the traveller would not go round once in twenty-four hours (which would be following the sun, and would cause the hour of the day to remain always the same), but in twenty-four hours and the best part of another hour; so that the day would seem to pass on, though very slowly, lasting a lunar month instead of a common day.

Every one who makes a long sea-voyage must have noted the importance attached to moon observations; and many are misled into the supposition that these observations are directly intended for the determination of the longitude (or, which is the same thing in effect, for determining true ship-time). This, however, is a mistake. The latitude can be determined at noon, as we have seen. A rough approximation to the local time can be obtained also, and is commonly obtained, by noting when the sun begins to dip after reaching the highest part of his course above the horizon. But this is necessarily only a rough approximation, and quite unsuited for determining the ship's longitude. For the sun's elevation changes very slowly at noon, and no dip can be certainly recognized, even from terra firma, far less from a ship, within a few minutes of true noon. A determination of time effected in this way serves very well for the ship's "watches," and accordingly when the sun, so observed, begins to dip, they strike "eight bells" and "make it noon." But it would be a serious matter for the crew if that was made the noon for working the ship's place; for an error of many miles would be inevitable.

The following passage from "Foul Play" illustrates the way in