The System of Copernicus

The ruin of Roman Empire produced the entire neglect of the study of Nature.

The empire of the Caliphs established the peace required by the sciences.

It was under the protection of those generous and magnificent princes, that the ancient philosophy and astronomy of the Greeks were restored and established in the East.

They translated the following Greek works into Arabic and studied them eagerly:

• Aristotle
• Ptolemy
• Hippocrates
• Galen

The victorious Saracens carried into Spain the learning and gallantry of the East, together with:

• the tables of Almamon
• the Arabian translations of Ptolemy and Aristotle

Thus Europe received a second time, from Babylon, the rudiments of Astronomy.

The motion of the Equalizing Circle represented the revolutions of the Celestial Spheres. But it was equable only when surveyed from a point that was different from their centers.

This introduced a real inequality into their motions, contrary to that most natural and fundamental idea which the former astronomers have aimed for.

This prompted Copernicus to create a new system that was more accurate than that of Ptolemy.

He rearranged the planets so that there would be regularity in their motions. To do this he looked back at all the principles of previous astronomers.

He found in Plutarch that some old Pythagoreans had represented the Earth as revolving around its own axis as the center of the universe.

Other Pythagoreans had removed it from the centre, and represented it as revolving in the Ecliptic like a star around the central fire.

He supposed that this central fire was the Sun.

It then occurred to him, that, if the Earth rotates every day around its axis, from west to east, then all the heavenly bodies would appear to revolve, in a contrary direction, from east to west.

The diurnal revolution of the heavens, upon this hypothesis, might be only apparent.

The firmament, which has no other sensible motion, might be perfectly at rest.

The Sun, the Moon, and the Five Planets, might have no other movement beside that eastward revolution, which is peculiar to themselves.

That, by supposing the Earth to revolve with the Planets, around the Sun, in an orbit, which comprehended within it the orbits of Venus and Mercury, but was comprehended within those of Mars, Jupiter, and Saturn, he could, without the embarrassment of Epicycles, connect together the apparent annual revolutions of the Sun, and the direct, retrograde, and stationary appearances of the Planets.

While the Earth really revolved round the Sun on one side of the heavens, the Sun would appear to revolve round the Earth on the other; that while she really advanced in her annual course, he would appear to advance eastward in that movement which is peculiar to himself.

That, by supposing the axis of the Earth to be always parallel to itself, not to be quite perpendicular, but somewhat inclined to the plane of her orbit, and consequently to present to the Sun, the one pole when on the one side of him, and the other when on the other, he would account for the obliquity of the Ecliptic; the Sun’s seemingly alternate progression from north to south, and from south to north, the consequent change of the seasons, and different lengths of days and nights in the different seasons.

If this new hypothesis thus connected together all these appearances as happily as that of Ptolemy, there were others which it connected together much better.

The 3 superior Planets, when nearly in conjunction with the Sun, appear always at the greatest distance from the Earth, are smallest, and least sensible to the eye, and seem to revolve forward in their direct motion with the greatest rapidity.

On the contrary, when in opposition to the Sun, that is, when in their meridian about midnight, they appear nearest the Earth, are largest, and most sensible to the eye, and seem to revolve backwards in their retrograde motion.

To explain these appearances, the system of Ptolemy supposed each of the these Planets to be at the upper part of their several Epicycles, in the one case; and at the lower, in the other.

But it afforded no satisfactory principle of connection, which could lead the mind easily to conceive how the Epicycles of those Planets, whose spheres were so distant from the sphere of the Sun, should thus, if one may say so, keep time to his motion.

The system of Copernicus afforded this easily, and like a more simple machine, without the assistance of Epicycles, connected together, by fewer movements, the complex appearances of the heavens. When the superior Planets appear nearly in conjunction with the Sun, they are then in the side of their orbits, which is almost opposite to, and most distant from the Earth, and therefore appear smallest, and least sensible to the eye. But, as they then revolve in a direction which is almost contrary to that of the Earth, they appear to advance forward with double velocity; as a ship, that sails in a contrary direction to another, appears from that other, to sail both with its own velocity, and the velocity of that from which it is seen.

On the contrary, when those Planets are in opposition to the Sun, they are on the same side of the Sun with the Earth, are nearest it, most sensible to the eye, and revolve in the same direction with it; but, as their revolutions round the Sun are slower than that of the Earth, they are necessarily left behind by it, and therefore seem to revolve backwards; as a ship which sails slower than another, though it sails in the same direction, appears from that other to sail backwards. After the same manner, by the same annual revolution of the Earth, he connected together the direct and retrograde motions of the two inferior Planets, as well as the stationary appearances of all the Five.

There are some other particular phaenomena of the two inferior Planets, which correspond still better to this system, and still worse to that of Ptolemy.

Venus and Mercury seem to attend constantly upon the motion of the Sun, appearing, sometimes on the one side, and sometimes on the other, of that great luminary; Mercury being almost always buried in his rays, and Venus never receding above forty–eight degrees from him, contrary to what is observed in the other three Planets, which are often seen in the opposite side of the heavens, at the greatest possible distance from the Sun. The system of Ptolemy accounted for this, by supposing that the centers of the Epicycles of these two Planets were always in the same line with those of the Sun and the Earth; that they appeared therefore in conjunction with the Sun, when either in the upper or lower part of their Epicycles, and at the greatest distance from him, when in the sides of them. It assigned, however, no reason why the Epicycles of these two Planets should observe so different a rule from that which takes place in those of the other three, nor for the enormous Epicycle of Venus, whose sides must have been forty–eight degrees distant from the Sun, while its center was in conjunction with him, and whose diameter must have covered more than a quadrant of the Great Circle. But how easily all these appearances coincide with the hypothesis, which represents those two inferior Planets revolving round the Sun in orbits comprehended within the orbit of the Earth, is too obvious to require an explanation.

Thus far did this new account of things render the appearances of the heavens more completely coherent than had been done by any of the former systems. It did this, too, by a more simple and intelligible, as well as more beautiful machinery. It represented the Sun, the great enlightener of the universe, whose body was alone larger than all the Planets taken together, as established immoveable in the center, shedding light and heat on all the worlds that circulated around him in one uniform direction, but in longer or shorter periods, according to their different distances. It took away the diurnal revolution of the firmament, whose rapidity, upon the old hypothesis, was beyond what even thought could conceive. It not only delivered the imagination from the embarrassment of Epicycles, but from the difficulty of conceiving these two opposite motions going on at the same time, which the system of Ptolemy and Aristotle bestowed upon all the Planets; I mean, their diurnal westward, and periodical eastward revolutions. The Earth’s revolution round its own axis took away the necessity for supposing the first, and the second was easily conceived when by itself. The Five Planets, which seem, upon all other systems, to be objects of a species by themselves, unlike to every thing to which the imagination has been accustomed, when supposed to revolve along with the Earth round the Sun, were naturally apprehended to be objects of the same kind with the Earth, habitable, opaque, and enlightened only by the rays of the Sun. And thus this hypothesis, by classing them in the same species of things, with an object that is of all others the most familiar to us, took off that wonder and uncertainty which the strangeness and singularity of their appearance had excited; and thus far, too, better answered the great end of Philosophy.