Tycho Brahe' System of Astronomy

September 30, 2022

Tycho Brahe introduced a new hypothesis that connected the irregularities in the motions of the Planets.

He completed his Treatise of Revolutions but was afraid of what people might think. And so he kept it in his closet for 30 years.

Finally in old age, he allowed it to be published but died as soon as it was printed.

It was almost immediately and universally disapproved of, by the learned as well as by the ignorant.

One of his disciples was Reinholdus.

  • He built astronomical tables called ‘Prutenic Tables’ that were more accurate than those in the Treatise of Revolutions.
  • These soon appeared to be corresponded more exactly with the heavens, than the Tables of Alphonsus.

This ought naturally to have formed a prejudice in favour of the diligence and accuracy of Copernicus in observing the heavens. But it ought to have formed none in favour of his hypothesis; since the same observations, and the result of the same calculations, might have been accommodated to the system of Ptolemy, without making any greater alteration in that system than what Ptolemy had foreseen, and had even foretold should be made.

It formed, however, a prejudice in favour of both. The learned begin to examine a hypothesis which afforded the easiest methods of calculation, and upon which the most exact predictions had been made.

The superior degree of coherence, which it bestowed upon the celestial appearances, the simplicity and uniformity which it introduced into the real directions and velocities of the Planets, soon disposed many astronomers, first to favour, and at last to embrace a system, which thus connected together so happily, the most disjointed of those objects that chiefly occupied their thoughts. Nor can any thing more evidently demonstrate, how easily the learned give up the evidence of their senses to preserve the coherence of the ideas of their imagination, than the readiness with which this, the most violent paradox in all philosophy, was adopted by many ingenious astronomers, notwithstanding its inconsistency with every sistem of physics then known in the world, and notwithstanding the great number of other more real objections, to which, as Copernicus left it, this account of things was most justly exposed.

It was adopted, however, nor can this be wondered at, by astronomers only. learned in all other sciences, continued to regard it with the same contempt as the vulgar.

Astronomers were divided about its merit. Many of them rejected it for contradicting the established system of Natural Philosophy.

They said that:

  • the Earth might really be moving even if its inhabitants were at rest.
  • the Sun, and Fixed Stars, might really be at rest
  • this is similar to a ship that sails through a smooth sea seems at rest to those who are in it, though really in motion, while the objects which she passes along, seem to be in motion, though really at rest.

The system of Copernicus gave motion to the Earth.

The adversaries of this hypothesis took pains to calculate the extreme rapidity of this motion.

The Earth’s circumference was 23,000 miles. If the Earth revolved everyday around its axis, every point of it near the equator would pass over above 23,000 miles in a day or 1,000 miles per hour or 16 miles per minute. This would be faster than a cannonball or sound.

Aristotle’s Philosophy fought this as a Natural and Violent motion.

Natural motion was that which flowed from an innate tendency in the body, as when a stone fell downwards. Violent motion, that which arose from external force, and which was, in some measure, contrary to the natural tendency of the body, as when a stone was thrown upwards, or horizontally.

No violent motion could be lasting. For, being constantly weakened by the natural tendency of the body, it would soon be destroyed.

The natural motion of the Earth was downwards, in a straight line to the center as that of fire and air was upwards.

It was the heavens only that revolved naturally in a circle. Neither, therefore, the supposed revolution of the Earth round its own center, nor that round the Sun, could be natural motions. They must therefore be violent, and consequently could be of that gravity was, no long continuance.

Copernicus replied that the heavens might have circular motion. But planets also had such a motion, but more rapid.

Neither Copernicus knew of modern mechanical principles.

The systems of Aristotle and Hipparchus supposed that the diurnal motion of the heavenly bodies to be infinitely more rapid than even that dreadful movement which Copernicus bestowed upon the Earth.

But they supposed, at the same time, that those bodies were objects of a quite different species, from any we are acquainted with, near the surface of the Earth, and to which, therefore, it was less difficult to conceive that any sort of motion might be natural.

Those objects, besides, had never presented themselves to the senses, as moving otherwise, or with less rapidity, than these systems represented them. The imagination, therefore, could feel no difficulty in following a representation which the senses had rendered quite familiar to it.

But when the Planets came to be regarded as so many Earths, the case was quite altered.

If the Earth revolved so rapidly from west to east, then:

  • a perpetual wind would set in from east to west, more violent than what blows in the greatest hurricanes.
  • a stone thrown westwards would fly to a much greater distance than one thrown with the same force eastwards;

as what moved in a direction, contrary to the motion of the Earth, would necessarily pass over a greater portion of its surface, than what, with the same velocity, moved along with it.

A ball dropped from the mast of a moving ship does not fall precisely at the foot of the mast, but behind it. In the same manner, a stone dropt from a high tower would not, upon the supposition of the Earth’s motion, fall precisely at the bottom of the tower, but west of it, the Earth being, in the mean time, carried away eastward from below it.

The Separation of Bodies

The followers of Copernicus tried to elude this objection, which, before the doctrine of the Composition of Motion had been explained was altogether unanswerable.

They allowed, that a ball dropped from the by Galileo, mast of a ship under sail would not fall at the foot of the mast, but behind it; because the ball, they said, was no part of the ship, and because the motion of the ship was natural neither to itself nor to the ball.

But the stone was a part of the earth, and the diurnal and annual revolutions of the Earth were natural to the whole, and to every part of it, and therefore to the stone.

The stone, therefore, having naturally the same motion with the Earth, fell precisely at the bottom of the tower.

Tycho Brache followed Copernicus’ philosophy answered that any such motion was natural to the whole Earth, yet the stone, which was separated from it could no longer be actuated by that motion.

The limb, which is cut off from an animal, loses those animal motions which were natural to the whole. The branch, which is cut off from the trunk, loses that vegetative motion which is natural to the whole tree.

Even the metals, minerals, and stones, which are dug out from the bosom of the Earth, lose those motions which occasioned their production and encrease, and which were natural to them in their original state. Though the diurnal and annual motion of the Earth, therefore, had been natural to them while they were contained in its bosom; it could no longer be so when they were separated from it.