It is probably safe to say that the prevailing conception of the lifework of both Tycho Brahe and Johannes Kepler derives almost completely from their respective roles in the discovery of the laws of elliptical motion. Given the significance of Kepler's triumph, and the notoriety of his account of it, it would be naive to expect anything else, and pedantic to insist on anything else. Still, it seems worthwhile to register a mild protest against the tendency to extrapolate from what is commonly

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... egarded as an essentially technical achievement to the conclusion that both Tycho and Kepler were essentially technicians -the one a juggler of hardware, the other a juggler of numbers. The insufficiency of this picture can be illustrated by reference to the work of each on the lunar theory. Tycho's place in the history of astronomy is that of an observer, par excellence ( e.g. Vinter Hansen 1942 ). This reputation is justified. So is Kepler s reputation as a theoretician. But the two do not logically combine to indicate that Tycho was deficient in theoretical matters. Certainly Tycho himself was ill-disposed toward admitting to any such limitations. There is no reason whatever to suppose that he collected planetary observations for thirty years with the intention, either conscious or unconscious, of turning them over to someone else for the final assessment. As it turned out, circumstances so dictated. But Tycho's efforts, though hampered by conflicting affairs and abbreviated by old age, were far from futile by any standard except Kepler's brilliant results. Kepler himself ( 1609, chap. 8 ) bears witness to Tycho's having accounted for Mars' longitudes within 2' -a knowledge which cannot have been an insignificant factor in Kepler's later refusal to be content with accounts that left 8' errors. Of course, the world has not considered itself the poorer because the task was left for Kepler to complete. There is no reason to quarrel with the spirit of Delambre's ( 1821 Vol. 1, p. xiv ) enthusiastic assessment of the situation: "If Tycho had remained on his island, Kepler would never have received his invitation; we would cer-* Presented at the Washington D. C. meeting of the American Association for the Advancement of Science, Dec. 26-29, 1966.