of the luminosity seen round the sun at the eclipses of 1560, 1567, and 1598.[1]
The planets had been, favourite objects with Tycho from his youth. His very first attempts at observing had been sufficient to show him how imperfectly the existing theories of the planetary motions agreed with the actually observed positions of the planets, and throughout his life he never neglected to take regular observations of the five planets.[2] His early observations of planets were of course similar to those made by his predecessors. The ancients had generally fixed the position of a planet by mere alignment, or, if the distance from a star was small, by expressing it in lunar diameters, while conjunctions of planets inter se or near approaches to fixed stars were greatly valued as tests of theory. As long as Tycho only possessed few and small instruments, he naturally often had recourse to these old methods, but he commenced also very early to adopt the method, first used by Walther, of measuring the distance of a planet from two well-known fixed stars.[3] At Hveen he never quite gave up this method, but he chiefly depended on meridian altitudes and observations with the armillæ, and even the difficult planet Mercury was carefully watched for and observed on every opportunity.[4]
Though Tycho did not live long enough to try his hand seriously at the theory of the planetary motions, we have
- ↑ Progym., p. 134; Kepler, Ad Vitell. Pared., chap. viii. (Opera, ii. p. 309); Riccioli, Almag. novum, ii. p. 372. See also Tycho's letter to Mästlin in 1598 (Opera, i. p. 46). About Tycho's observations of the solar and lunar diameters, see above, chapter viii. p. 191.
- ↑ In a letter to Rothmann (Epist., p. 114) Tycho expressed his regret that so little attention was paid to the planets at Cassel, since the positions of fixed stars were principally of interest by enabling an observer to follow the course of the planets.
- ↑ At first the youthful observer generally only measured the distance from one star; but from December 1564 two stars are often, and from 1569 always employed.
- ↑ The earliest observation of Mercury seems to be of April 17, 1574, at Heridsvad.
