Discovery of Gravity

This series has six easy 5 minute installments. This first installment: An Apple and a Moon.

Introduction
This series was written before Albert Einstein altered science theories of gravity forever. While Newtonian physics no longer monopolizes the science, it remains an important part of it. It leveled up the “new learning” in the post-renaissance.

David Brewster wrote extensively on scientific subjects.

This selection is from Memoirs of the Life, Writings and Discoveries of Sir Isaac Newton by Sir David Brewster published in 1855. For works benefiting from the latest research see the “More information” section at the bottom of these pages.

Sir David Brewster (1781-1868) was a British scientist, inventor, author, and academic administrator.

Time: 1666
Place: Woolsthrope, England

Many admirers of Sir Isaac Newton have asserted that his was the most gigantic intellect ever bestowed on man. He discovered the law of gravitation, and by it explained all the broader phenomena of nature, such as the movements of the planets, the shape and revolution of the earth, the succession of the tides. Copernicus had asserted that the planets moved, Newton demonstrated it mathematically.

His discoveries in optics were in his own time almost equally famous, while in his later life he shared with Leibnitz the honor of inventing the infinitesimal calculus, a method which lies at the root of all the intricate marvels of modern mathematical science.

Newton should not, however, be regarded as an isolated phenomenon, a genius but for whom the world would have remained in darkness. His first flashing idea of gravitation deserves perhaps to be called an inspiration. But in all his other labors, experimental as well as mathematical, he was but following the spirit of the times. The love of science was abroad, and its infinite curiosity. Each of Newton’s discoveries was claimed also by other men who had been working along similar lines. Of the dispute over the gravitation theory Sir David Brewster, the great authority for the career of Newton, gives some account. The controversy over the calculus was even more bitter and prolonged.

It were well, however, to disabuse one’s mind of the idea that Newton’s work was a finality, that it settled anything. As to why the law of gravitation exists, why bodies tend to come together, the philosopher had little suggestion to offer, and the present generation knows no more than he. Before Copernicus and Newton men looked only with their eyes, and accepted the apparent movements of sun and stars as real. Now, going one step deeper, we look with our brains and see their real movements which underlie appearances. Newton supplied us with the law and rate of the movement — but not its cause. It is toward that cause, that great “Why?” that science has ever since been dimly groping.

In the year 1666, when the plague had driven Newton from Cambridge, he was sitting alone in the garden at Woolsthrope, and reflecting on the nature of gravity, that remarkable power which causes all bodies to descend toward the center of the earth. As this power is not found to suffer any sensible diminution at the greatest distance from the earth’s center to which we can reach — being as powerful at the tops of the highest mountains as at the bottom of the deepest mines — he conceived it highly probable that it must extend much further than was usually supposed. No sooner had this happy conjecture occurred to his mind than he considered what would be the effect of its extending as far as the moon. That her motion must be influenced by such a power he did not for a moment doubt; and a little reflection convinced him that it might be sufficient for retaining that luminary in her orbit round the earth.

Though the force of gravity suffers no sensible diminution at those small distances from the earth’s center at which we can place ourselves, yet he thought it very possible that, at the distance of the moon, it might differ much in strength from what it is on the earth. In order to form some estimate of the degree of its diminution, he considered that, if the moon be retained in her orbit by the force of gravity, the primary planets must also be carried round the sun by the same power; and by comparing the periods of the different planets with their distances from the sun he found that, if they were retained in their orbits by any power like gravity, its force must decrease in the duplicate proportion, or as the squares of their distances from the sun. In drawing this conclusion, he supposed the planets to move in orbits perfectly circular, and having the sun in their center. Having thus obtained the law of the force by which the planets were drawn to the sun, his next object was to ascertain if such a force emanating from the earth, and directed to the moon, was sufficient, when diminished in the duplicate ratio of the distance, to retain her in her orbit.

In performing this calculation it was necessary to compare the space through which heavy bodies fall in a second at a given distance from the center of the earth, viz., at its surface, with the space through which the moon, as it were, falls to the earth in a second of time while revolving in a circular orbit. Being at a distance from books when he made this computation, he adopted the common estimate of the earth’s diameter then in use among geographers and navigators, and supposed that each degree of latitude contained sixty English miles.

In this way he found that the force which retains the moon in her orbit, as deduced from the force which occasions the fall of heavy bodies to the earth’s surface, was one-sixth greater than that which is actually observed in her circular orbit. This difference threw a doubt upon all his speculations; but, unwilling to abandon what seemed to be otherwise so plausible, he endeavored to account for the difference of the two forces by supposing that some other cause must have been united with the force of gravity in producing so great velocity of the moon in her circular orbit. As this new cause, however, was beyond the reach of observation, he discontinued all further inquiries into the subject, and concealed from his friends the speculations in which he had been employed.

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