of many planets with their satellites and countless asteroids, only an approximation is possible.  The actual motions as observed and measured from year to year are most complex.  Can these be completely accounted for by the mutual attractions of the bodies, according to the law of gravitation as enunciated by Sir Isaac Newton?  In Newcomb’s words, “Does any world move otherwise than as it is attracted by other worlds?” Of course, Newcomb has not been the only astronomer at work on this problem, but it has been his life-work and his contributions to its solution have been very noteworthy.

It is difficult to make the ordinary reader understand the obstacles in the way of such a determination as this.  Its two elements are, of course, the mapping out of the lines in which the bodies concerned actually do move and the calculations of the orbits in which they ought to move, if the accepted laws of planetary motion are true.  The first involves the study of thousands of observations made during long years by different men in far distant lands, the discussion of their probable errors, and their reduction to a common standard.  The latter requires the use of the most refined methods of mathematical analysis; it is, as Newcomb says, “of a complexity beyond the powers of ordinary conception.”  In works on celestial mechanics a single formula may fill a whole chapter.

This problem first attracted Newcomb’s attention when a young man at Cambridge, when by analysis of the motions of the asteroids he showed that the orbits of these minor planets had not, for several hundred thousand years past, intersected at a single point, and that they could not, therefore, have resulted, during that period, from the explosion of a single large body, as had been supposed.

Later, when Newcomb’s investigations along this line had extended to the major planets and their satellites, a curious anomaly in the moon’s motion made it necessary for him to look for possible observations made long before those hitherto recorded.  The accepted tables were based on observations extending back as far as 1750, but Newcomb, by searching the archives of European observatories, succeeded in discovering data taken as early as 1660, not, of course, with such an investigation as this in view, but chiefly out of pure scientific curiosity.  The reduction of such observations, especially as the old French astronomers used apparent time, which was frequently in error by quarter of an hour or so, was a matter of great difficulty.  The ancient observer, having no idea of the use that was to be made of his work, had supplied no facilities for interpreting it, and “much comparison and examination was necessary to find out what sort of an instrument was used, how the observations were made, and how they should be utilized for the required purpose.”  The result was a vastly more accurate lunar theory than had formerly been obtained.

During the period when Newcomb was working among the old papers of the Paris Observatory, the city, then in possession of the Communists, was beset by the national forces, and his studies were made within hearing of the heavy siege guns, whose flash he could even see by glancing through his window.