four hours, carrying all the planets with it. The sun is the centre of the orbits of the five planets, of which Mercury and Venus move in orbits whose radii are smaller than that of the solar orbit, while the orbits of Mars, Jupiter, and Saturn encircle the earth. This system accounts for the irregularities in the planetary motions which the ancients explained by epicycles and Copernicus by the annual motion of the earth, and it shows why the solar motion is mixed up in all the planetary theories.[1] The remaining inequalities, which formerly were explained by the excentric circle and the deferent, and by Copernicus by epicycles moving on excentric circles, could also, in the new hypothesis, be explained in a similar way. As the planets are not attached to any solid spheres, there is no absurdity in letting the orbits of Mars and the sun intersect each other, as the orbits are nothing real, but only geometrical representations.
This is all which Tycho considered it necessary to set forth about his system in the book on the comet, but he stated his intention of giving a fuller account of it on a future occasion, which never came. We shall finish our account of his labours connected with the comet of 1577 before we consider his system a little more closely.
The comet was by Tycho supposed to move round the sun in an orbit outside that of Venus, and in the direction opposite to that of the planets, the greatest elongation from the sun being 60°. He was unable to represent the observed places of the comet by a uniform motion in this orbit, and was obliged to assume an irregular motion, slowest when in inferior conjunction, increasing when the comet was first discovered, and afterwards again decreasing. Tycho remarks
- ↑ This alludes to the circumstance, which had appeared so strange to the ancients, that the period of the motion of each upper planet in its epicycle was precisely equal to the synodical period of the planet, while in the case of the two inferior planets the period in the deferent in the Ptolemean system was equal to the sun's period of revolution.
