Introduction

We mainly owe the “discovery” of the fact that the Milky Way system is a galaxy to the work of Shapley, Lindblad, and Oort. Shapley (1918a) noted that “we may say confidently that the plane of the Milky Way is also a symmetrical plane in the great system of globular clusters.” He (Shapley 1918b) also found that “The center [of this system of globular clusters], which lies in the region of the rich star clouds of Sagittarius near the boundary of Scorpio and Ophiuchus, has the coordinates R.A. = 17h 30m, Decl.= –30°.” Shapley concluded that “A consideration of the foregoing results leads naturally to the conclusion that globular clusters outline the extent and arrangement of the total [G]alactic organization.” Shapley's discovery may be thought of as the second Copernican revolution. Copernicus had shifted the center of the Universe from the Earth to the Sun, and Shapley took the even greater step of moving it from the solar system to the center of the Galaxy. Subsequently Lindblad (1927) and Oort (1927, 1928) were able to show that (a) the Galactic disk is in rapid differential rotation about a center that coincides with that of Shapley's globular cluster system and that (b) globular clusters and high-velocity stars rotate more slowly around the same center than does the Galactic disk. The notion that the Milky Way system is a spiral galaxy received its ultimate confirmation when Morgan, Whitford & Code (1953) were able to outline three Galactic spiral-arms in the vicinity of the Sun. A year later, the existence of global spiral-structure in the Milky Way was demonstrated by the 21-cm observations of Westerhout (1954) and Schmidt (1954).