 The situation changed completely when Kepler came into contact at Prague with Tycho’s observations (which, as Kepler often reports, were seldom at his disposal). However, a change of attitude is evident in AN, where he used Tycho’s observations without restriction (which is something he makes clear in the work’s title). In part 2 (chap. 7–21), he presents the “vicarious hypothesis”, which in the end he refutes. This hypothesis represents the best result which can be reached within the limits of traditional astronomy. This works with circular orbits and with the supposition that the motion of a planet appears regular from a point on the lines of apsides. Against the traditional method, here, Kepler does not cut the eccentricity into equal parts but leaves the partition open. To check his hypothesis, he needs observations of Mars in opposition, where Mars, the Earth, and the Sun are at midnight on the same line. From Tycho, he “inherited” ten such observations between the years 1580 and 1600, and to them he added another two for 1602 and 1604. In chapters 17–21, Kepler carries out an observational and computational check of his vicarious hypothesis. On the one hand, he points out that this hypothesis is good enough, since the variations of the calculated positions from the observed positions fall within the limits of acceptability (2 minutes of arc). In fact, Kepler presents this hypothesis as the best hypothesis which can be proposed within the framework of a “traditional astronomy”, as opposed to his new astronomy, which he will offer in the following parts of the work. On the other hand, this hypothesis can be falsified if one takes the observations of the latitudes into consideration. Further calculations with these observations produce a difference of eight minutes, something that cannot be assumed because the observations of Tycho are reliable enough. Kepler’s famous sentence runs: “these eight minutes alone will have led the way to the reformation of all of astronomy” (AN, KGW 3, p. 286; Engl. trans., p. 286). There seems to be agreement that Kepler’s AN contains the first explicit consideration of the problem of observational error (for this question see Hon 1987 and Field 2005).