[1.]
The average eccentricity of the orbits of the four
inner planets (per unit mass) is 0.0221, and of the four outer planets
is 0.0489.
[2.]
It would seem that all rotating nebulæ should in
reality possess some of the attributes of spiral motion. Whether the
spiral structure should be visible or invisible to a terrestrial
observer would depend upon the sizes and distances of the nebulæ,
upon the distribution of materials composing them, and perhaps upon
other factors. See developed the hypothesis that spiral nebulæ owe
their origin to the collision of two nebulæ. Collisions of this
kind could readily occur because of the enormous dimensions of the
nebulæ, and motions of rotation and consequently spiral structure
might readily result therefrom. The abnormally high speeds of the spiral
nebulæ are apparently a very strong objection to the
hypothesis.
[3.]
Bohlin found a parallax of 0"17 for the Andromeda Nebula,
and Lampland thinks that Nebula N.G.G. 4594 has a proper motion of approximately 0"05
per annum.
