140 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



miles, and another faint star Lalande 21,185 at about 47 million 

 million miles. Thus our nearest neighbors among the stars are at 

 almost exactly a million times the distances of our nearest neighbors 

 among the planets. After these comes Sirius, the brightest star in 

 the sk}', at 50 million million miles. From here on there is a steady 

 succession of objects until we reach distances of more than 20,000 

 times that of Sirius; but long before these distances are reached 

 other objects, spiral and spheroidal nebulae, and ultimately star 

 clusters, are foimd to be mingled with the stars. The furthest 

 object the distance of which is known with any accuracy is the 

 star cluster N. G. C. 7006, which Shapley estimates to be 25,000 

 times as distant as Sirius. This cluster is so remote that its light 

 takes 200,000 years to reach us; even for light to cross the cluster 

 takes hundreds of years. To all appearances the star cloud N. G. C. 

 6822 is still more remote. According to Shapley, its distance is 

 about 6 million million million miles, a distance which light takes 

 a million years to traverse. So far as is known at present, this 

 brings us to the end of our universe, or perhaps I ought to say it 

 brings us back to the beginning. 



It is no easy matter to get all these different distances clearly 

 into focus simultaneously, but let us try. The earth speeds round 

 the sun at about twenty miles a second; in a year it describes an 

 orbit of nearly 600 million miles circumference. If we represent 

 the earth's orbit by a pinhead or a full stop of radius one-hundredth 

 of an inch, the sun will be an invisible speck of dust, and the earth 

 an ultramicroscopic particle one-millionth of an inch in diameter. 

 Neptune's orbit, which incloses the whole of the solar system, will 

 be represented by a circle the size of a threepenny piece, while the 

 distance to the nearest star, Proxima Centauri, will be about 75 

 yards and that to Sirius about 160 yards. On this same scale the 

 distance to the remote star cluster N. G. C. 7006 is 2,400 miles and 

 that to the star cloud N. G. C. 6822 about 12,000 miles, so that 

 roughly speaking the whole universe may be represented by our 

 earth. 



It thus appears that we are on this occasion to discuss the origin 

 and past history of a system which bears the same relation to the 

 universe as a whole as does a threepenny piece to our earth. "Why 

 are we so interested in this particular threepenny piece? Primarily 

 because, although a poor thing, it is our own, or at least one particle 

 of it, one-millionth of an inch in diameter, is our own. But there is 

 a historical reason of a less sentimental kind. We have already 

 noticed the immensity of the gap between our system and its nearest 

 neighbors. As regards astronomical knowledge this gap has taken 

 a o-reat deal of crossing. Well on into last century, human knowl- 



