Page:The Solar System - Six Lectures - Lowell.djvu/136



To account for so orderly an arrangement Laplace supposed:—

a. That the matter now composing our solar system was once in the form of a nebula.

b. That this original nebula was very hot, a fire-mist.

c. That it possessed initially a slow rotation.

d. That as it contracted under its own gravity and thus, from the principle of conservation of moment of momentum, rotated faster as it shrank, it rotated always like a solid body with the same angular velocity throughout, until its outer portions, which went the fastest, came to go so fast that the centrifugal tendency overcame the centripetal force and they were left behind as a ring.

e. That this ring revolved as a whole until it broke, rolled back upon itself and made a planet; the outer parts of the ring having the swiftest motions, the resulting planet rotated in the same sense that it revolved.

f. The planet thus formed gave birth in like manner to its satellite system.

The prestige of Laplace gave this explanation hypothesis a mental momentum which has carried conviction nearly to the present day. But it is erroneous for all that, nor can it be made to work by any additions or slight alterations as some text-books will