An estimate is made of the maximum range over which the motion of a particle about the Sun and in the galactic field is stable in Hill's sense. An equation fitting the Hill surface is found, with allowance made for the fact that the galactic potential differs from that of a point mass. The volume enclosed by the surface is 3.35 cubic parsecs, the greatest distance from the Sun being 1.42 parsecs, this being along the line to the centre of the Galaxy.

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