On the shape of cometary halos

There is yet no consensus on the shape of halos. The question as to what curve describes best the halos has significant meaning for model-building and for the understanding of the dynamics of halos. By means of a modified Newtonʹs formula of hydrodynamics, it is shown that the shape of cometary halos is close to a catenary, rather than the paraboloidal envelopes of dust grain orbits discussed by Eddington.