THE LAW OF EQUILIBRIUM 149 



idea has been a subject of speculation. But to the eye of 

 Newtonian philosophy the sine qua non is, the presence 

 of a central force. From time to time they have con- 

 sidered the eligibility of such great stars as Sirius, 

 Canopus, Arcturus, Alcyone, Orion, and even certain 

 star clusters, as hitching posts to which to tether the uni- 

 verse; never once suspecting the cosmic virtues inherent 

 in the stellar resultant. At the present time the consensus 

 of astronomical opinion appears to echo that of Doctor 

 W. W. Campbell (v. p. 12, ante), namely, that the path 

 of the sun is to all intents and purposes rectilinear and 

 that "the curvature of our path is undoubtedly so slight 

 that we may consider it as a straight line for many gen- 

 erations of astronomers to come." 



So minute is the range of the annual revolution of the 

 earth around the sun relatively to the major orbit of the 

 latter, that you can readily see why the earth's axis, 

 year by year, should remain (almost) parallel to itself, 

 the secret of our change of seasons. The slight departure 

 from parallel that it does suffer in one such revolution is, 

 of course, according to the new hypothesis, reflected in 

 the alteration of the projected position of the Vertex on 

 the precessional circle traced on the sky. 



Observe, furthermore, that in revolving around the 

 sun, the earth is obliged to pass twice each year across 

 the precise line of his path, once in front of him, when 

 she is on her way out, and the second time behind him, 

 when she is on her way in. The first of these two points 

 is known as the vernal equinox, and the second as the 

 autumnal equinox. When the earth is at her outermost 

 point, you will see that her Arctic region is pointed sun- 

 ward, so that then we of the north have summer, while 

 our winter solstice occurs when the earth is innermost. 



In the very nature of things, with every star in move- 

 ment, there is no such thing as an absolutely fixed point 

 or an unalterable direction. Astronomers understand 

 this (though strangely forgetful of it at times!) and do 

 the best they can, under the circumstances, by choosing 

 the most stable points and using them for reference. As 



