ASTRONOMY. 213 



through these investigations, which, no doubt, mark the beginning of 

 a new era in our knowledge of the past history of the solar system : 



1. The lunar period must have been shorter in the past, and may be 

 traced back from the present 27.3 days, until initially the Moon revolved 

 round the Earth in from two to four hours. 



2. The inclination of the orbit to the "proper plane" must have 

 been larger in the past, and may be traced back from the present 5^ 9' 

 until it was G^ or 7°. This was a maximum inclination, and in the 

 more remote past the inclination was less and initially" was very small 

 or zero. 



3. The inclination of the proper plane to the ecliptic must have been 

 greater in the past, and may be traced back from its present 8" until it 

 was in very early times about 11° 45'. It is possible that initially this 

 inclination was less, and that the 11° 45' was a maximum value. 



4. The eccentricity of the orbit must have been smaller in the past. 

 Either at one time it had a minimum value, before which it had a 

 maximum value, and, again, earlier it was very small or zero, or else 

 the maximum value never occurred, aud the eccentricitj^ has always 

 been increasing. The history of the eccentricity depends on the nature 

 of the tides in the Earth, but the former of these alternatives seems 

 the more i)robable. 



At the present time the Earth rotates in twenty-four hours, its equa- 

 tor is inclined at an angle of 9" to a plane which Mr. Darwin calls 

 "the proper plane of the Earth" (the mean equator). This proper 

 plane is inclined at an angle of 23° 28' to the ecliptic, and its intersec- 

 tion with the ecliptic is the equinoctial line. He finds that the fric- 

 tional tides in the Earth have caused changes which may be summa- 

 rized as follows : 



5. The day must have been shorter in the past, and it may be traced 

 back from its present value of twenty-four hours until initially it was 

 from two to four hours in length. It was then identical with the 3Ioon's 

 Ijeriod of revolution as described in (1). 



G. The inclination of the equator to the Earth's proper ])lane must 

 have been larger in the past, and may be tra<ied back from the present 

 value of 9" until it was about 2° 45'. This was a maximum iuclination, 

 and in the more remote past the inclination was less, and initially it 

 was very small or zero. 



7. The inclination of the Earth's proper plane to the ecliptic must 

 have been smaller in the past, and may be traced back from its present 

 value of 23° 28' until initially it was 11° 45', or, perhajis, somewhat less. 

 It was then identical with the proper plane of the lunar orbit, and this 

 is true whether or not 11° 45' was a maximum inclination of the lunar 

 proper plane to the ecliptic as described in (1). 



The preceding statements may be subject to varieties of detail, ac- 

 cording to the nature of the tides raised in the Earth, but the above is 

 a summary of what appears to be the most probable course of evolution. 



