CHAPTER I 
THE ASTRONOMICAL SURVEY OF THE UNIVERSE 
1. The moon, our nearest neighbour in the sky, is 240,000 miles away 
from us; a distance which light, travelling at 186,000 miles a second, traverses 
in a little over one second. The farthest astronomical objects whose distances 
are known are so remote that their light takes over one hundred million years 
to reach us. The ratio of these two periods of time—a hundred million years 
to a second—is the ratio of the greatest to the least distance with which the 
astronomer has to deal, and within this range of distances lie all the objects 
of his study. 
As he wanders through this vast range with the aid of his telescope, he 
finds that the great majority of the objects he encounters fall into well-defined 
classes; they may almost be said to be “manufactured articles” in the sense 
in which Clerk Maxwell applied the phrase to atoms. Just as atoms of 
hydrogen or of oxygen are believed to be of similar structure and properties 
wherever they occur in nature, so the various astronomical objects—common 
stars, binary stars, variable stars, star-clusters, spiral nebulae, etc.—are believed 
to be, to a large extent at least, similar structures no matter where they occur. 
The similarity, it is true, is not so definite or precise as that between the 
atoms of chemistry, and perhaps a better comparison is provided by the 
different species of vegetation which inhabit a country. Plants and trees, 
while differing in size, vigour, age and secondary characteristics, nevertheless 
fall into clearly-defined species. Basing our metaphor on this, we may say 
that recent extensions in telescopic power have revealed no new species of 
astronomical objects, but have merely multiplied the numbers of examples 
of objects which belong to known species. For this reason we may suppose 
that we are already acquainted with the principal species of astronomical 
objects in the universe. 
The task of the observational astronomer is to survey and explore the 
universe, and to describe and classify the various types of objects of which it 
is constituted, discovering what law and order he may in their observed 
arrangement and behaviour. But only the dullest of human minds can rest 
content with a mere catalogue of observed facts; the alert mind asks always 
for the why and the wherefore. How comes it that these various classes of 
objects exist, but no others? What is the relation between them? Does one 
of them for instance, produce, or transform into, others? If so, what is the 
sequence of these changes? How did this universe of objects begin, and what 
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