Page:Tycho brahe.djvu/387

results. From the distance measures of the comet of 1577 Woldstedt found the probable error of one observed distance = ± 4′.2,^[1] but as he mixed the sextant measures with those obtained with the cross-staff, which Tycho always mentions as an untrustworthy instrument, this large probable error is not surprising. The most valuable investigation which we possess concerning Tycho's instruments is the discussion of the observations of the comet of 1585 by C. A. F. Peters.^[2] When this comet appeared, Tycho's collection of instruments was complete, and we may assume that the observations are typical. Tycho states that his indications of time have been corrected by the observed hour-angles of stars, and by recomputing these the mean correction of + 22^s.5 was found, with a probable error of ± 37^s. This only shows, as Tycho merely gave the time in whole minutes, that the great armillæ of Stjerneborg were well adjusted. But a very much better proof of this is furnished by the observations. By the armillæ the comet was compared in right ascension with certain standard stars, while its declination was observed with the same instrument. From the total of these observations Peters found that the polar axis of the armillæ was inclined to the horizon by an angle which exceeded the latitude by only 65″ ± 33″, and formed with the meridian an angle of only 36″ ± 13″. The probable error of one observation of declination was ± 49″, that of one right ascension = 81″, and consequently that of one observed hour-angle = ± 57″. The error of collimation (or parallax, as Tycho called it) was − 30″.1, by which amount the observed declinations were too large. The comet was also observed with the sextans trigonicus, and the probable error of one observed distance was found

1. F. Woldstedt, De gradu praecisionis positionum cometæ 1577. Helsingfors, 1844.
2. Astron. Nachr., xxix. p. 209 et seq. (1849).