"[T]he high perfection to which solar astronomy has been able to elevate itself... is... essentially due to... all the particular, and... accidental facilities presented by the peculiarly favourable constitution of our planetary system. The planets... are quite few in number, and their masses... very unequal, and much less than that of the sun; they are... very distant from one another; they have forms almost spherical; their orbits are nearly circular, and only slightly inclined to each other, and so on. It results from all these circumstances that the perturbations are generally inconsiderable, and that... it is usually sufficient to take into the account, in connexion with the action of the sun... the influence of only one other planet..."