Читать книгу Practical Talks by an Astronomer - Harold Jacoby - Страница 5

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A short time ago the writer had occasion to rummage among the archives of the Royal Astronomical Society in London, to consult, if possible, the original manuscripts left by one Stephen Groombridge, an English astronomer of the good old days, who died in 1832. It was known that they had been filed away about a generation ago, by the late Sir George Airy, who was Astronomer Royal of England between the years 1835 and 1881. After a long search, a large and dusty box was found and opened. It was filled with documents, of which the topmost was in Sir George's own handwriting, and began substantially as follows:

"List of articles within this box.

"No. 1, This list,

"No. 2, etc., etc."

Astronomical precision can no further go: he had listed even the list itself. Truly, Airy was rightly styled "prince of precisians." A worthy Astronomer Royal was he, to act under the royal warrant of Charles II., who established the office in 1675. Down to this present day that warrant still makes it the duty of His Majesty's Astronomer "to apply himself with the most exact care and diligence to the rectifying of the tables of the motions of the heavens and the places of the fixed stars, in order to find out the so much desired longitude at sea, for the perfecting the art of navigation."

The "so much desired longitude at sea" is, indeed, a vastly important thing to a maritime nation like England. And only in comparatively recent years has it become possible and easy for vessels to be navigated with safety and convenience upon long voyages. The writer was well acquainted with an old sea-captain of New York, who had commanded one of the earliest transatlantic steamers, and who died only a few years ago. He had a goodly store of ocean yarn, fit and ready for the spinning, if he could but find someone who, like himself, had known and loved the ocean. In his early sea-going days, only the wealthiest of captains owned chronometers. This instrument is now considered indispensable in navigation, but at that time it was a new invention, very rare and costly. Upon a certain voyage from England to Rio Janeiro, in South America, the old captain could remember the following odd method of navigation: The ship was steered by compass to the southward and westward, more or less, until the skipper's antique quadrant showed that they had about reached the latitude of Rio. Then they swung her on a course due west by compass, and away she went for Rio, relying on the lookout man forward to keep the ship from running ashore. For after a certain lapse of time, being ignorant of the longitude, they could not know whether they would "raise" the land within an hour or in six weeks. We are glad of an opportunity to put this story on record, for the time is not far distant when there will be no man left among the living who can remember how ships were taken across the seas in the good old days before chronometers.

Anyone who has ever been a passenger on a great transatlantic liner of to-day knows what an important, imposing personage is the brass-bound skipper. A very different creature is he on the deck of his ship from the modest seafaring man we meet on land, clad for the time being in his shore-going togs. But the captain's dignity is not all brass buttons and gold braid. He has behind him the powerful support of a deep, delightful mystery. He it is who "takes the sun" at noon, and finds out the ship's path at sea. And in truth, regarded merely as a scientific experiment, the guiding of a vessel across the unmarked trackless ocean has few equals within the whole range of human knowledge. It is our purpose here to explain quite briefly the manner in which this seeming impossibility is accomplished. We shall not be able to go sufficiently into details to enable him who reads to run and navigate a magnificent steamer. But we hope to diminish somewhat that small part of the captain's vast dignity which depends upon his mysterious operations with the sextant.

To begin, then, with the sextant itself. It is nothing but an instrument with which we can measure how high up the sun is in the sky. Now, everyone knows that the sun slowly climbs the sky in the morning, reaches its greatest height at noon, and then slowly sinks again in the afternoon. The captain simply begins to watch the sun through the sextant shortly before noon, and keeps at it until he discovers that the sun is just beginning to descend. That is the instant of noon on the ship. The captain quickly glances at the chronometer, or calls out "noon" to an officer who is near that instrument. And so the error of the chronometer becomes known then and there without any further astronomical calculations whatever. Navigators can also find the chronometer error by sextant observations when the sun is a long way from noon. The methods of doing this are somewhat less simple than for the noon observation; they belong to the details of navigation, into which we cannot enter here.

Incidentally, the captain also notes with the sextant how high the sun was in the sky at the noon observation. He has in his mysterious "chart-room" some printed astronomical tables, which tell him in what terrestrial latitude the sun will have precisely that height on that particular day of the year. Thus the terrestrial latitude becomes known easily enough, and if only the captain could get his longitude too, he would know just where his ship was that day at noon.

We have seen that the sextant observations furnish the error of the chronometer according to ship's time. In other words, the captain is in possession of the correct local time in the place where the ship actually is. Now, if he also had the correct time at that moment of some well-known place on shore, he would know the difference in time between that place on shore and the ship. But every traveller by land or sea is aware that there are always differences of time between different places on the earth. If a watch be right on leaving New York, for instance, it will be much too fast on arriving at Chicago or San Francisco; the farther you go the larger becomes the error of your watch. In fact, if you could find out how much your watch had gone into error, you would in a sense know how far east or west you had travelled.

Now the captain's chronometer is set to correct "Greenwich time" on shore before the ship leaves port. His observations having then told him how much this is wrong on that particular day, and in that particular spot where the ship is, he knows at once just how far he has travelled east or west from Greenwich. In other words, he knows his "longitude from Greenwich," for longitude is nothing more than distance from Greenwich in an east-and-west direction, just as latitude is only distance from the equator measured in a north-and-south direction. Greenwich observatory is usually selected as the beginning of things for measuring longitudes, because it is almost the oldest of existing astronomical establishments, and belongs to the most prominent maritime nation, England.

One of the most interesting bits of astronomical history was enacted in connection with this matter of longitude. From what has been said, it is clear that the ship's longitude will be obtained correctly only if the chronometer has kept exact time since the departure of the ship from port. Even a very small error of the chronometer will throw out the longitude a good many miles, and we can understand readily that it must be difficult in the extreme to construct a mechanical contrivance capable of keeping exact time when subjected to the rolling and pitching of a vessel at sea.

It was as recently as the year 1736 that the first instrument capable of keeping anything like accurate time at sea was successfully completed. It was the work of an English watchmaker named John Harrison, and is one of the few great improvements in matters scientific which the world owes to a desire for winning a money prize. It appears that in 1714 a committee was appointed by the House of Commons, with no less a person than Sir Isaac Newton himself as one of its members, to consider the desirability of offering governmental encouragement for the invention of some means of finding the longitude at sea. Finally, the British Government offered a reward of $50,000 for an instrument which would find the longitude within sixty miles; $75,000, if within forty miles, and $100,000, if within thirty miles. Harrison's chronometer was finished in 1736, but he did not receive the final payment of his prize until 1764.

We shall not enter into a detailed account of the vexatious delays and official procedures to which he was forced to submit during those twenty-eight long years. It is a matter of satisfaction that Harrison lived to receive the money which he had earned. He had the genius to plan and master intricate mechanical details, but perhaps he lacked in some degree the ability of tongue and pen to bring them home to others. This may be the reason he is so little known, though it was his fortune to contribute so large and essential a part to the perfection of modern navigation. Let us hope this brief mention may serve to recall his memory from oblivion even for a fleeting moment; that we may not have written in vain of that longitude to which his life was given.