154 FROM NEBULA TO NEBULA 



more distant still, and so on, rising higher and higher in 

 scale, and bearing iipon their twigs systems, and upon 

 their branches systems of systems of worlds. To my 

 mind, the flatness and the well-proved depth of the disc of 

 the Milky Way eloquently testify that it is a gravitational 

 unit in a mightier invisible universe, invisible only be- 

 cause the light of its component stars is drowned in the 

 vast sea of space. As such a unit, of course, the Galaxy 

 should rotate around an axis in search of equilibrium, and 

 it appears that astronomers have already detected some 

 indications of such rotation! Underlying the whole 

 structural scheme of the macrocosm, as under the hum- 

 blest habitation of man, lies the great principle that New- 

 ton took such pains to eliminate EQUILIBRIUM. 



THE OBLIQUITY OF THE ECLIPTIC 



Inter-related with this problem of precession is that 

 of the change in the obliquity of the ecliptic. To quote 

 from Young again (Genl. Astr., p. 137): "The ecliptic 

 slightly and very slowly shifts its position among the 

 stars, thus altering the latitudes of the stars and the 

 angle between the ecliptic and the equator, i. e., the ob- 

 liquity of the ecliptic. This obliquity is at present about 

 24' less than it was 2,000 years ago, and is still decreasing 

 about half a second a year. It is computed that this di- 

 minution will continue for about 15,000 years, reducing 

 the obliquity to 22% , when it will begin to increase. The 

 whole change, according to J. Herschel, can never exceed 

 about 1 20' on each side of the mean." 



The explanation of this phenomenon is not far to 

 seek. We have only to assume the plane, of the grav- 

 Antarctic Circle to constitute the ' ' invariable ' ' plane of 

 the solar system and to picture the oval of the earth's 

 orbit as, so to speak, balanced on the rim of this Circle in 

 such a way as constantly to preserve to it an inclination 

 of 1 20' throughout the 260 centuries that it takes for it 

 to ' ' slide ' ' around the precessional circle. Or, to put the 

 matter from another point of view, the revolution of the 

 ecliptic around the solar orbit causes its pole (the Ver- 



