Kepler’s love of harmony encouraged him to continue his pursuits, in spite of the most mortifying disappointments, until he discovered that admirable relation which subsists between the periodic times of the primary planets, and their distances from the sun; the squares of the former being as the cubes of the latter. This discovery was of great importance to the perfection of Astronomy; because the periods of the planets are more easily found by observation, and from them their several relative distances may be determined with great accuracy by this rule. He likewise found from observation, that the planets do not move in circles; but in elipses, having the sun in one focus. But the causes lay hid from him, and it was left as the glory of Sir Isaac, to demonstrate that both these things must necessarily follow from one simple principle, which almost every thing in this science tends to prove does really obtain in Nature: I mean, that the planets are retained in their orbits by forces directed to the sun; which forces decrease as the squares of their distances encrease.
Kepler also discovered that the planets do not move equally in their orbits, but sometimes swifter, sometimes slower; and that not irregularly, but according to this certain rule; That in equal times, the areas described by lines drawn from the planet to the sun’s centre, are equal. This, Sir Isaac likewise demonstrated must follow, if the planet be retained in its orbit by forces directed to the sun, and varying with the distance in any manner whatsoever. These three discoveries of Kepler, afterwards demonstrated by Newton, are the foundation of all accuracy in astronomical calculations.[[A19]]
We now come to that great discovery, which lay concealed from the most subtle and penetrating geniuses amongst mankind, until these latter ages; which so prodigiously enlarged the fields of astronomy, and with such rapidity handed down one curiosity after another, from the heavens to astonished mortals, that no one capable of raising his eyes and thoughts from the ground he trod on, could forbear turning his attention, in some degree, to the subject that engages us this evening.
Galileo, as he himself acknowledges, was not the first inventor of the telescope, but he was the first that knew how to make a proper use of it.[[A20]] If we consider that convex and concave lenses had been in use for some centuries, we shall think it probable that several persons might have chanced to combine them together, so as to magnify distant objects; but that the small advantage apparently resulting from such a discovery, either on account of the badness of the glasses or the unskilfulness of the person in whose hands they were, occasioned it to be neglected.
But Galileo, by great care in perfecting his telescope, and by applying a judicious eye, happily succeeded; and with a telescope magnifying but thirty times, discovered the moon to be a solid globe, diversified with prodigious mountains and vallies, like our earth; but without seas or atmosphere. The sun’s bright disk, he found frequently shaded with spots, and by their apparent motions proved it to be the surface of a globe, revolving on its axis in about five and twenty days. This it seems was a mortifying discovery to the followers of Aristotle; who held the sun to be perfect without spot or blemish.[[A21]] Some of them, it is said, insisted that it was but an illusion of the telescope and absolutely refused to look through one, lest the testimony of their senses should prove too powerful for their prejudices.
Galileo likewise discovered the four attendants of Jupiter, commonly called his satellites:[[A22]] Which at first did not much please that great ornament of his age, the sagacious Kepler. For by this addition to the number of the planets, he found their Creator had not paid that veneration to certain mystical numbers and proportions, which he had imagined. Let us not blush at this remarkable instance of philosophical weakness, but admire the candour of the man who confessed it.
Galileo not only discovered these moons of Jupiter, but suggested their use in determining the longitude of places on the earth; which has since been so happily put in practice, that Fontenelle does not hesitate to affirm, that they are of more use to Geography and Navigation,[[A23]] than our own moon. He discovered the phases of Mars and Venus; that the former appears sometimes round and sometimes gibbous, and that the latter puts on the shapes of our moon: And from this discovery, he proved to a demonstration, the truth of the Copernican System.[[A24]] Nor did that wonderful ring, which surrounds Saturn’s body, without touching it, and which we know nothing in nature similar to, escape his notice; though his telescope did not magnify sufficiently to give him a true idea of its figure.
Amongst the fixed stars too, Galileo pursued his enquiries. The Milky-Way, which had so greatly puzzled the ancient Philosophers, and which Aristotle imagined to be vapours risen to an extraordinary height, he found to consist of an innumerable multitude of small stars; whose light appears indistinct and confounded together to the naked eye. And in every part of the heavens, his telescope shewed him abundance of stars, not visible without it. In short, with such unabated ardour did this great man range through the fields of Astronomy, that he seemed to leave nothing for others to glean after him.
Nevertheless, by prodigiously encreasing the magnifying powers of their telescopes, his followers made several great discoveries; some of which I shall briefly mention. Mercury was found to become bisected, and horned near its inferior conjunction, as well as Venus. Spots were discovered in Mars, and from their apparent motion, the time of his revolution on an axis nearly perpendicular to its orbit, was determined. A sort of belts or girdles, of a variable or fluctuating nature, were found to surround Jupiter, and likewise certain spots on his surface, whence he was concluded to make one revolution in about ten hours on his axis; which is likewise nearly perpendicular to his orbit. Five[[A25]] moons or satellites were found to attend Saturn, which Galileo’s telescope; on account of their prodigious distance, could not reach:[[A26]] And the form of his ring was found to be a thin circular plane, so situated as not to be far from parallel to the plane of our equator; and always remaining parallel to itself. This ring, as well as Saturn, evidently derives its light from the sun, as appears by the shadows they mutually cast on each other.
Besides several other remarkable appearances, which Hugenius[[A27]] discovered amongst the fixed stars, there is one in Orion’s Sword, which, I will venture to say, whoever shall attentively view, with a good telescope and experienced eye, will not find his curiosity disappointed. “Seven small stars, (says he,) of which three are very close together, seemed to shine through a cloud, so that a space round them appeared much brighter than any other part of heaven, which being very serene and black looked here as if there was an opening, through which one had a prospect into a much brighter region.” Here some have supposed old night to be entirely dispossessed, and that perpetual daylight shines amongst numberless worlds without interruption.