But it is quite possible that what appear to be parabolic orbits may, in reality, be ellipses of very great eccentricity. The difficulty in determining the precise shape of a comet's orbit arises from the fact that all three of the curves just mentioned closely approximate to one another in the neighbourhood of their common focus, the sun, and it is only in that part of their orbits that comets are visible. The whole question is yet in abeyance, but, as we have said, it seems likely that all comets really move in elliptical orbits, and consequently never get entirely beyond the control of the sun's attraction. But in all cases the orbits of comets are much more eccentric than those of the large planets. The famous comet of Halley, for instance, which has the longest period of any of the known periodical class, about seventy-five years, is 3,293,000,000 miles from the sun when in aphelion, and only 54,770,000 miles when in perihelion.