Summary 33 



arose by the rotational separation of successive rings 

 from a contracting nebula. This theory is confronted 

 by serious difficulties; thus the residual sun is not 

 rotating so quickly as it ought to do if Laplace's 

 theory is right. Moreover, the equatorial plane of 

 the sun ought to lie precisely in the mean plane of the 

 planets' orbits, whereas it is inclined seven degrees 

 to such a mean plane. 



3. The characteristic nebula in the heavens to-day 

 is the spiral nebula, consisting of a central nucleus 

 and spirally-twisted ejected arms emerging sym- 

 metrically from opposite ends. Condensations on 

 these arms might form stars, systems of stars, but 

 the separated stars would not proceed to repeat the 

 process, thus forming planets. According to the 

 experts a solar system like ours could not arise in 

 this way. 



4. Thus has arisen the tidal hypothesis, that the 

 sun, arising perhaps from the condensation of a 

 nebula, or as a knot on a spiral arm, might come to be 

 subject to intense tidal forces because of the near 

 approach of a passing star. A great jet or secondary 

 nebula drawn out beyond recall might break into 

 fragments and condense into knots which became the 

 planets and our earth. Some of the sun's prominences 

 to-day rise to heights of nearly 300,000 miles. 



5. Residual materials of the arms might be sub- 

 sequently gathered into the knots, along with more 

 dust-like particles called planetesimals, which are 

 regarded by Chamberlin as making important con- 

 tributions to the young earth. 



