2 SCIENCE PROGRESS 



only be necessary to know the motion ot each star. For the 

 brighter stars we have fairly accurate observations of their 

 positions which date back 160 years, and from comparisons 

 with modern observations their proper motions — i.e. their 

 apparent motions in the sky — can be determined with great 

 accuracy. For the majority of the fainter stars we have not, 

 however, sufficiently distant observations for determining 

 accurately their proper motions. The measurement of the 

 velocities of stars in the line of sight is also one of the most 

 recent developments of astronomy, so that our knowledge of 

 these is still in its early stages. As we cannot hope for some 

 time to obtain sufficient information thus to solve the problem 

 directly, it becomes necessary to get such indirect information 

 as we can, and bound up with the problem of the present 

 structure is the additional and more difficult problem of the 

 evolution of the universe — more difficult because to mark its 

 changes we must measure not by hundreds but by millions 

 of years. 



The questions which even a very cursory examination of the 

 problem raises are very numerous. Is our universe finite or 

 infinite in extent ? Can we, with the aid of our telescopes, 

 penetrate to its extremities, and number the stars ? If it is 

 finite, are there other stellar universes existing outside our own, 

 and if so, in what relation does ours stand to them ? How has 

 our universe been evolved, what will be its end, and how long 

 its duration? What is its form and where is its centre? To 

 some of these questions we can give answers with more or 

 less certainty, but to others of them we cannot yet reply. 



In all these investigations the Milky Way holds a position 

 of fundamental importance. Some part of the Milky Way may 

 be seen on any clear night in the year, but it is seen best 

 in early winter, when it passes near our zenith in the evening. 

 It is a broad, luminous stream of faint stars, with many branches 

 and dark rifts, but on the whole lying very nearly in a plane 

 which is inclined at a few degrees to the ecliptic, and which 

 intersects the celestial equator in the constellations of Aquila 

 and Monoceros. It is important as being the plane of symmetry 

 of the stellar universe, and the coordinates which express the 

 position of a star relative to it are called its galactic longitude 

 and latitude, the longitude being measured eastward from the 

 point of its intersection with the celestial equator in Aquila. 



