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ASTRONOMY: H. SHAPLEY 
simply through plotting (as in figure 1) the galactic longitude and the 
distance projected on the galactic plane. No striking lack of sym- 
metry appears in this diagram, except the almost total absence of bright 
open clusters in the first 90° of galactic longitude. 
The mean distance of the 70 open clusters along the plane is 5900 
parsecs, all individual values (except for the Pleiades) lying between 
400 and 16,000 parsecs. By taking the dip of the central hne of the 
Milky Way^^ as 1°, and the distance of the sun above the plane as 60 
parsecs,^^ the distance of the stars and star clouds that enter Newcomb's 
90* 
FIG. 1. DISTRIBUTION OF OPEN CLUSTERS IN THE GALACTIC PLANE 
The direction angles are galactic longitudes; the annuli are 25,000 parsecs in width 
(visual) determination of the position of the galactic circle is of the 
order of 60/sin 1° = 3500 parsecs. Although the uncertainty of this 
value is large, it seems reasonable to infer that the open clusters are 
intermingled with the non-cluster stars of the galactic stratum. 
The diagram in figure 2 illustrates, for all the open clusters, globular 
clusters, and Cepheid variables falling between galactic longitudes 290° 
and 360°, the distances projected on the galactic plane plotted as 
abscissae against the distances from the plane as ordinates. This 
region of the sky, containing the great Milky Way clouds of Ophiuchus, 
