The combined effects of the stars and of Jupiter appear to determine the main statistical features of the orbits of comets. - Jan Hendrik Oort

The article indicates how three facts concerning the long-period comets, which hitherto were not well understood, namely the random distribution of orbital planes and of perihelions, and the preponderance of nearly-parabolic orbits, may be considered as necessary consequences of the perturbations acting on the comets.

The distribution of galaxies is clumpy on all scales, ranging from binaries, triples, and multiples, through groups containing a few and a hundred galaxies, to rich clusters with thousands of members, masses up to 10^15±1M_☉, and diameters of the order of 10 Mpc. The latter have been discussed by Neta A. Bahcall in a preceding volume of this series. The clusters are at the end of the scale of the more or less regular structures in the Universe.

From a score of well-observed original orbits it is shown that the "new" long-period comets generally come from regions between about 50000 and 150000 A.U. distance. The sun must be surrounded by a general cloud of comets with a radius of this order, containing about 10¹¹ comets of observable size; the total mass of the cloud is estimated to be of the order of 1/10 to 1/100 of that of the earth. Through the actlllllllllthis cloud into the vicinity of the sun.