Nat the Navigator. A Life of Nathaniel Bowditch. For Young Persons

by Henry I. Bowditch

HALLEY’S COMET.

GRANDEUR OF MAN.

MOON’S MOTIONS.

We must now try to give you a short account of the “Mécanique Céleste,” and of Dr. Bowditch’s labors upon it. The original work consists of five volumes, but Dr. Bowditch lived to finish the translation of and commentary upon only the first four. There are about fifteen hundred pages in the original, while there are three thousand eight hundred and eighteen in the American translation. The object of the original work may be known from the following introductory remarks by La Place, on the occasion of printing the first volume, in 1798: “Newton, towards the end of the last century, published his discovery of the laws of gravity, or of the power by which the solar system is held together. Since that period, geometricians have succeeded in bringing under this law all the known phenomena of the system of the universe. I mean to bring together those scattered themes and facts upon this subject, so as to form one whole, which shall embrace all the known results of gravity upon the motions, forms, &c., of the fluid and solid bodies that compose our solar system, as well as of those other similar systems that are spread around in the immensity of space.” You probably all understand from this quotation the general object of the “Mécanique Céleste.” La Place likewise informs us that the work is divided into two parts. In the first he proposes to give the methods for determining the motions of the heavenly bodies, their forms, the motions of the oceans and seas upon their surfaces, and finally the movements of rotation of these spheres about their own axes. In the second part, he promises to apply the rules which he has given in the first to the planets and the satellites which move around them, and likewise to the comets. The first part is found in the first two volumes, the second part occupies the last two. From these few remarks you will perceive the immense task imposed upon himself by La Place, and at the same time the grandeur of it. How wonderful, that a simple man can attempt to mark out the course of the heavenly bodies, which we see clustering around us at night! But how much more wonderful does man become, when we find he has the power to foretell to us the return of comets that have never been seen by any one living now—comets that have been, during our lives, travelling into the far-off fields of space! Strange that a simple man can prophesy, to a day, their return! Many of us now living remember a beautifully bright and clear comet, which in 1835 appeared, as had been predicted, after an absence of seventy-six years. It was called Halley’s comet, after its first discoverer. At first it seemed like a bright speck in the heavens towards the north; but the next night it was larger. It seemed to approach, with fearful rapidity, from evening to evening, and, sweeping in majesty across our western sky, disappeared gradually in its progress towards the sun, around which it whirled, and again appeared, more faintly visible than before, just over our eastern horizon, as if to give us one more glimpse of itself, a strange messenger of the Almighty, before it passed off on its far-distant journey, not to return until those who were then young and free as air, are all laid quietly in the grave, or have become enfeebled and decrepit by the approach of age. Truly, great is God, who made the comet; but to me man also seems full of grandeur, when I find him capable of even foretelling the exact passage of such a body. Yet La Place enables any man to prophesy this; and in his “Mécanique Céleste” we may find all the methods of investigation necessary for this object. But he likewise tells us the forms of the planets; he enables us to measure the ring which surrounds the planet Saturn, and enables us to decide, at least in some degree the form and mass of the sun. In this same work he treats of those curious phenomena, which, as we see them daily, we think of little moment—the flow and ebb of the sea, or, in other words, high and low tides,—and the causes of them. He treats of the motion of the earth about its centre, and of the same motions in the moon and planets. These are the chief objects of the first and second volumes. The third volume, as we have already hinted, contains questions of great intricacy, and of immense importance; namely, the exact motions of the planets around the sun, as affected by all the attractions exerted upon them by the various bodies of the universe; and the still more important motions of our moon around the earth. I say important, because the exact knowledge of the course of this body is of the greatest moment to every sailor who attempts to go from one country to another over the trackless ocean. By means of observations upon this planet, the seaman can sail over distant waters for many months, and be able to return, when he may wish, to his own home in safety. Hence the importance of the astronomer to the simple navigator of our planet. The history of Dr. Bowditch is another proof of the truth of this statement. By his accurate knowledge of astronomy, by his ability to follow La Place in his investigations of all the motions of the solar system, he was enabled to produce a work on navigation which is sought for wherever the English language is spoken, as it combines the best methods of using the results of pure astronomy in the art of navigation. The “Practical Navigator” would never have maintained its hold upon the community as it has done, if Dr. Bowditch had not been as skilful in mathematics and astronomy as in the details of navigation.

JUPITER’S SATELLITES.

PLANET NEPTUNE.

But to return to the “Mécanique Céleste.” The fourth volume contains similar investigations, namely, the motions of the satellites, or moons, about the other planets. Our moon’s motions about the earth, and the revolutions of Jupiter’s satellites are the most important. Jupiter has four satellites. These were the first that the invention of the telescope by Galileo revealed to man; and by their frequent revolutions around the planet, they have in their turn shown to us many of the laws which govern the whole planetary system, besides many curious and interesting facts in regard to their own forms and masses. From the eclipses or disappearances of the first satellite, when it passes on the side of the planet opposite to that at which the observer from the earth is looking, it has demonstrated the velocity of light. Finally, the author treats of the seven moons, or satellites, of Saturn, and likewise of those of the planet Herschel, about which much less is known.[12]

POWERS ACTING UPON COMETS.

After attending to these subjects, La Place investigates the powers which act upon comets, which tend to turn from their courses those bodies, which, as I have before said, are flying in very many directions throughout the universe, and which are liable to be moved out of their direction by the actions of some planets near which they may come. This was the case with a comet in 1770, whose course was wholly changed by the planet Jupiter drawing it towards its own body. To investigate the various laws of these disturbing forces is one subject of this volume. Some other subjects are treated of, but of these I shall now not speak.

NOTES ON MÉCANIQUE CÉLESTE.