THE EINSTEIN-BESSO MANUSCRIPT ON THE MOTION OF THE PERIHELION OF MERCURY I The manuscript, written jointly by Einstein and his lifelong friend Michele Besso, deals with the perihelion motion of Mercury[1] on the basis of the "Entwurf" theory.[2] Internal and external evidence suggest that the two men closely collaborated on this problem during a visit by Besso with Einstein in Zurich in June 1913. The bulk of the manuscript was presumably written during this visit. Einstein may have added some material later in 1913; some of the Besso material is certainly from later, most likely from early 1914. From early on in his search for a new relativistic theory of gravitation, Einstein was interested in the problem of Mercury's perihelion. In a letter to a friend in 1907,[3] Ein- stein had already expressed his hope that such a theory would explain the anomalous advance of Mercury's perihelion. After this letter, however, the Mercury anomaly is not mentioned again, neither in Einstein's published papers nor in surviving correspon- dence, until late 1915.[4] On 18 November 1915 Einstein presented a paper[5] to the Prussian Academy of Sciences showing that his new, still developing general theory of relativity yields a perihelion advance for Mercury of 43" per century, in striking agreement with observation. The success carried over unproblematically to the final version of the theory published shortly thereafter.[6] When Einstein subsequently reported on his new theory to Sommerfeld[7] and Lorentz,[8] he mentioned that one of the reasons for abandoning the 1913 "Entwurf" theory was that it failed to explain the Mercury anomaly.[9] In the letter to Sommer- feld, he explicitly mentioned that the "Entwurf" theory predicts a perihelion advance for Mercury of 18" per century. This is also the number given by Droste in a paper published in December 1914.[10] The manuscript under discussion reveals that Einstein actually did the calculation himself, shortly after he and Grossmann had finished the "Entwurf" paper[11] in early 1913. The expression Einstein and Besso arrived at for [1]For a discussion of the history of the problem of the anomalous advance of the perihelion of Mercury, see Roseveare 1982. [2]See the editorial note, "Einstein on Gravitation and Relativity: The Collaboration with Marcel Grossmann," pp. xx, for more on this theory. [3]Einstein to Conrad Habicht, 24 December 1907 (Vol. 5, Doc. 69). [4]There is a reference to calculations on perihelion motion in a letter to Erwin Freundlich, dated 30 September 1915. [5]Einstein 1915d. [6]Einstein 1915e. [7]Einstein to Arnold Sommerfeld, 28 November 1915. [8]Einstein to H.A. Lorentz, 1 January 1916. [9]See Norton 1984, sec. 7. [10]Droste 1914, p. 1010. [11]Einstein and Grossmann 1913 (Doc. 13).