DEVELOPMENT OF THE PLANETARIUM — CHAMBERLAIN 265 



globes contain the projectors of the fixed stars, one globe for the 

 northern hemisphere of the sky, one for the southern. The lantern 

 slides, or diapositives, are so shaped that their images fit together 

 to make a complete picture of the starry heavens. 



The main structure, containing all the projectors, is so mounted 

 that it may turn independently about any one of three axes. First, 

 it may turn about an axis parallel to the polar axis of the earth. 

 When this motion is used without other motions, the effect naturally 

 is to transport the images across the dome sky in exactly the same 

 way that the daily rotation of the earth on its axis apparently 

 moves the real bodies across our sky each 24 hours. 



Second, the machine may rotate about an axis perpendicular to 

 the plane in which the earth moves about the sun. Without the 

 other motions in use, the effect of this is to swing the north pole 

 of the sky around the circle that it makes each 26,000 years with 

 the precessional "wobbling" of the earth's axis. Thus one can go 

 backward or forward in time. For example, the lecturer can set 

 the instrument back some 5,000 years to 3,000 B. C. when Alpha 

 Draconis was our North Star. Or, by putting the instrument ahead 

 some 12,000 years, we see Vega marking the north pole of the 

 heavens, and the Southern Cross visible from the latitude of New 

 York. The axis of this precessional motion of the instrument inter- 

 sects the daily-motion axis at the center of the room. 



Third, through this same intersection runs the axis for the remain- 

 ing motion of the machine, a horizontal one from the east to the 

 west point. Rotation about it transports the images on the dome 

 as if the viewer of the skies were traveling along a meridian of the 

 earth from pole to pole. This is used to demonstrate the changed 

 appearance of the skies from different latitudes of the earth, so 

 that one may go to the Land of the Midnight Sun, or to the North 

 Pole, and observe the apparent movement of sun, moon, and stars 

 from there. Or, traveling south, one may see the Magellanic 

 Clouds, Canopus, and the Southern Cross. 



The heavy moving parts of the machine are carried on a 

 light but carefully built steel latticework. 



The whole apparatus has several different speeds, all of which 

 are many times faster than the real motions. This makes it pos- 

 sible to condense a very long astronomical story, so that anyone 

 can get a clear understanding in a few minutes of the seemingly 

 intricate, though actually simple, workings of the heavenly bodies. 



Nearby objects such as the planets and the sun and moon, which 

 appear to move against the background of the stars from day to 

 day, are represented by separate projectors having independent 



