4 
The Astronomical Survey of the Universe [ch. i 
true, have substantially greater inclinations and eccentricities than are found 
among the great planets. For instance, the planet whose orbit has the greatest 
inclination to the general plane (invariable plane) of the solar system is 
Mercury with an inclination of 7° O'; against this the orbit of the minor 
planet Pallas has an inclination of 34° 43', and that of Hidalgo an inclination 
of 43°. The planet whose orbit shews the greatest eccentricity is Mercury, 
with an eccentricity of 0 - 206, whereas the minor planets Albert and Hidalgo 
have eccentricities of 0o4 and 065 respectively. 
But the outstanding fact remains that all the orbits are described in 
the same direction. Adopting an argument which Laplace advanced in his 
Systeme du monde (1796), we may remark that if the directions of motion 
of 2000 planets and minor planets were a matter of pure chance, the odds 
would be 2 19 " — 1 to one against the coincidence of the orbits being all de 
scribed in the same direction. Thus the odds that the directions of the orbits 
are not a matter of pure chance, are far greater than those in favour of well- 
attested historical events: it is more certain that some definite cause underlies 
the directions of motion in these orbits than it is, for instance, that the 
Athenians won the battle of Marathon, or that Queen Anne is dead. 
The motion of the satellites of the planets continues, on the whole, the 
story of ordered arrangement already told by the motions of the planets, 
although here certain definite exceptions must be noted. These exceptions 
are limited to the outermost edges of the solar system and the outermost edges 
of the systems of Jupiter and Saturn. They are as follows: 
Neptune, the outermost planet, has only one satellite, and this moves with 
retrograde motion—i.e. in the direction opposite to that in which Neptune 
and the other planets move round the sun. 
Uranus, the next outermost planet, has four satellites, all moving in the 
equatorial plane of the planet, which is highly inclined to the general plane 
of the solar system. 
Saturn, which comes next, has nine satellites, the outermost of which 
(Phoebe) revolving at a mean distance of 217 diameters of Saturn, moves 
with retrograde motion. 
Jupiter has nine satellites, the two outermost of which move with retro 
grade motion. 
The Distances of the Stars. 
The nearer Stars. 
3 . The solar system has occupied the foreground of our picture of the 
universe, because its members are incomparably nearer to us than other 
astronomical bodies. As a preliminary to filling in the rest of the picture let 
us imagine the various objects in the universe arranged in the order of their 
distances from the earth. Disregarding bodies much smaller than the earth,