seen that he was at Benatky occupied with the theory of Mars, and succeeded in representing the longitudes well (Kepler says within 2′), while the latitudes gave more trouble.[1] But already at Uraniborg he had not contented himself with a mere accumulation of material, but had drawn some conclusions from the comparison of his results with the tabular places of the planets. We have seen that Tycho, like Ptolemy and Copernicus, assumed the solar orbit to be simply an excentric circle with uniform motion. But already in 1591, he might have perceived from the motion of Mars that this could not be sufficient, as he wrote to the Landgrave that "it is evident that there is another inequality, arising from the solar excentricity, which insinuates itself into the apparent motion of the planets, and is more perceptible in the case of Mars, because his orbit is much smaller than those of Jupiter and Saturn."[2] He concluded (strangely enough) that his own planetary system alone could account for this, and he can therefore not have had a clear idea of the cause of the phenomenon. Again, in his letter to Kepler of April 1, 1598, he mentioned that the annual orbit of Mars (according to Copernicus) or the epicycle of Ptolemy was not always of the same size with regard to the excentric, but varied to the extent of 1° 40′.[3] This eventually led Kepler to the discovery of the elliptic orbits, but it showed him already in Tycho's lifetime that the solar excentricity was only half as great as hitherto supposed, and that the remainder of the equation of centre would have to be accounted for by a uniform motion round a punctum æquans (that is, as long as only circular orbits were
- ↑ Above, p. 303.
- ↑ Epist. Astr., p. 206. Magini had also noticed this apparent inequality in Mars; see above, chap. ix. p. 213. Tycho also alludes to it in Mechanica, fol. Gr. 3: "Tum quoque circuitum illum annuum, quem Copernicus per motum Terræ in orbe magno, veteres secundum Epicycles excusarunt, variationi cuidam obnoxium esse perspeximus."
- ↑ Opera, i. p. 44, iii. p. 267 (De Stella Martis, xxii.).
