Tycho Brahe' System of Astronomy

Tycho Brache was the great restorer and advancer of Astronomy.

• He spent his life and wasted his fortune on it.
• His observations were both more numerous and more accurate than those of all the astronomers who had gone before him.
  • He himself was so much affected by objections to the point that he could not embrace the system of Copernicus

He admired Copernicus and all his astronomical observations tended to confirm his system. They demonstrated, that:

• Venus and Mercury were sometimes above, and sometimes below the Sun
• consequently, the Sun, and not the Earth, was the center of their periodical revolutions
• Mars, when in his meridian at midnight, was nearer to the Earth than the Earth is to the Sun; though, when in conjunction with the Sun, he was much more remote from the Earth than that luminary

These were absolutely inconsistent with the system of Ptolemy. Copernicus’ system proved that:

• the Sun, and not the Earth, was the center of the periodical revolutions of Mars, Venus, and Mercury
• the Earth was placed between the orbits of Mars and Venus.
• the same thing was probable with regard to Jupiter and Saturn

• Comets were superior to the Moon, and moved through the heavens in all possible directions
  • This was incompatible with the Solid Spheres of Aristotle and Purbach, and the established Astronomy.

Tycho created a new hypothesis from:

• these observations
• his aversion to the system of Copernicus

It kept:

• the Earth as the immoveable center of the universe
• the firmament revolved every day around the Earth from east to west and, by some secret virtue, carried the Sun, the Moon, and the 5 Planets with it

But Tycho gave them a contrary periodical eastward revolution of their own.

• This made them appear to be left behind by the Firmament every day.
• The Sun was the center of the periodical revolutions of the 5 Planets; the Earth, that of the Sun and Moon.

The 5 Planets followed the Sun in its revolution around the Earth, as they did the Firmament in its diurnal rotation.

The 3 superior Planets comprehended the Earth within the orbit in which they revolved round the Sun.

Each of them had an Epicycle to connect together, in the same way as in the system of Ptolemy, their direct, retrograde, and stationary appearances.

Despite their immense distance, they followed the Sun in its revolution around the Earth.

They kept always at an equal distance from the sun.

They were brought much nearer to the Earth when in opposition to the Sun, than when in conjunction with it.

Mars, the nearest of them, when in his meridian at midnight, came within the orbit which the Sun described round the Earth, and consequently was then nearer to the Earth than the Earth was to the Sun.

The appearances of the two inferior Planets were explained, in the same manner, as in the system of Copernicus, and consequently required no Epicycle to connect them.

The circles in which the 5 Planets performed their periodical revolutions round the Sun, as well as those in which the Sun and Moon performed theirs round the Earth, were, as both in the old and new hypothesis, Eccentric Circles.

• These connected together their differently accelerated and retarded motions.

Tycho’s system combined the systems of Ptolemy and Copernicus.

It was better than Ptolemy’s system because it accounts for the motions of the 2 inferior Planets.

It was more complex by supposing the different revolutions of the 5 Planets to be performed around 2 different centers:

1. The diurnal around the Earth
2. The periodical round the Sun

It was more complex and more incoherent than that of Copernicus.

Those who considered the heavens only, favoured the system of Copernicus.

• This connected all the appearances.

But that those who looked on the Earth adopted Tycho’s system which:

• left the Earth in the center of the universe
• did less violence to the imagination.

The learned were sensible of the intricacy and of the many incoherences of Tycho’s system.

It did not explain:

• why the Sun, Moon, and 5 Planets, should follow the revolution of the Firmament
• why the 5 Planets, despite the immense distance of the 3 superior ones, should obey the periodical motion of the Sun
• why the earth, placed between the orbits of Mars and Venus, should remain immoveable in the center of the Firmament, and constantly resist the influence of whatever carried bodies that were so much larger than itself, and that were placed on all sides of it, periodically round the Sun.

Tycho Brahe died before he had fully explained his system.

The objection to the system of Copernicus was at last fully answered by Galileo:

• 30 years after the death of Tycho
• around 100 years after the death of Copernicus

Galileo explained the nature of the composition of motion.

He showed, both from reason and experience, that a ball dropt from the mast of a ship under sail would fall precisely at the foot of the mast.

• This removed the principal objection against the system of Copernicus

Copernicus left the Moon to revolve round the Earth as before.

But no example of any such secondary Planet having then been discovered in the heavens, there seemed still to be this irregularity remaining in the system.

Galileo, who first applied telescopes to discovered, by their assistance, the Satellites of Jupiter, which, revolving Astronomy, round that Planet, at the same time that they were carried along with it in its revolution, round either the Earth, or the Sun, made it seem less contrary to the analogy of nature, that the Moon should both revolve round the Earth, and accompany her in her revolution round the Sun.

It had been objected to Copernicus, that, if Venus and Mercury revolved round the Sun, in an orbit comprehended within the orbit of the Earth, they would show all the same phases with the Moon, present, sometimes their darkened, and sometimes their enlightened sides to the Earth, and sometimes part of the one, and part of the other.

His telescopes rendered the phases of Venus quite sensible, and thus Galileo.

The mountains and seas, which, by the help of the same instrument, he discovered, or imagined he had discovered in the Moon, rendering that Planet, in every respect, similar to the Earth, made it seem less contrary to the analogy of nature, that, as the Moon revolved round the Earth, the Earth should revolve round the Sun.

The spots which, in the same manner, he discovered in the Sun, demonstrating, by their motion, the revolution of the Sun round his axis, made it seem less improbable that the Earth, a body so much smaller than the Sun, should revolve round her axis in the same manner.

Succeeding telescopical observations, discovered, in each of the Five Planets, spots not unlike those which Galileo had observed in the Moon, and thereby seemed to demonstrate what Copernicus had only conjectured, that the Planets were naturally opaque, enlightened only by the rays of the Sun, habitable, diversified by seas and mountains, and, in every respect, bodies of the same kind with the Earth; and thus added one other probability to this system.

By discovering, too, that each of the Planets revolved round its own axis, at the same time that it was carried round either the Earth or the Sun, they made it seem quite agreeable to the analogy of nature, that the Earth, which, in every other respect, resembled the Planets, should, like them too, revolve round its own axis, and at the same time perform its periodical motion round the Sun.