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352 standard declinations (after correcting them for refraction) = ± 25″.9.

It is interesting to see that observations of absolute right ascension were made at Cassel about the same time, and by the same method, except that Jupiter was at first used instead of Venus. As Jupiter could not be observed with the sun above the horizon, this involved trusting to the rate of the clocks for many hours, which perhaps was more feasible at Cassel, where Bürgi introduced the use of the pendulum for controlling the clocks. In 1587 Venus was, however, made use of, the altitude and azimuth of the sun, Venus, and Aldebaran being observed in succession. The results thus found for the right ascension of the latter star agreed well inter se, fixing it at 63° 10′ for the beginning of 1586, or more than 6′ greater than that found by Tycho. This systematic error, with which all the right ascensions determined by means of Aldebaran became affected, and which also, with nearly the same amount, entered into the longitudes, was discussed in several letters between the Landgrave, Rothmann, and Tycho. The Landgrave thought 5′ or 6′ a very trifling quantity, not worth mentioning, as nobody hitherto had been able to determine longitudes with that accuracy. Tycho at first suggested that the discrepancy might be caused by an error in the solar declination, caused by a faulty suspension of the plumbline which marked the zero point on the quadrant at Cassel, and to which Rothmann had referred in a former letter. Afterwards he concluded that the error was caused by all the observations being made in the evening, when refraction