XPost: rec.arts.sf.science   
   From: max@alcyone.com   
      
   Russell Wallace wrote:   
      
   > I'd ideally like to point it at the sun. Then the nearside cap could   
   > be coated in solar cells while the rest of the surface is painted   
   > black to help radiate heat.   
   >   
   > But in that case, it seems that a quarter orbit later it'll be side on   
   > to the sun, another quarter orbit the opposite end will point at the   
   > sun etc, since conservation of angular momentum will tend to keep the   
   > spin axis pointing in the same direction relative to the rest of the   
   > universe, not relative to the sun.   
   >   
   > Is there any way to change that (I mean, reasonable ways, i.e. without   
   > expending propellant or using huge gyroscopes etc)?   
      
   No, there isn't.   
      
   > For example,   
   > Earth's axis precesses every 26,000 years IIRC; how does that square   
   > with conservation of angular momentum?   
      
   Because the Earth is not a uniform sphere (it bulges at the equator),   
   the force due to gravity from the Sun applies a torque on the Earth.   
   That torque changes the Earth's angular momentum.   
      
   Since the Sun is applying a torque, this changes the angular momentum of   
   the Earth, and results in precession.   
      
   > Is there a way a habitat's axis   
   > could be made to "precess" through a full circle every year?   
      
   The only way would be applying a large, constant external torque.  (Your   
   habitat would likely precess due to solar gravity as well, but on   
   timescales far too long to be useful for you.)   
      
   > Or if not, is the best solution then to orient it vertically, let most   
   > of the surface be mirror colored, and paint the end caps black?   
      
   By "vertically" I presume you mean so that the axis is parallel to the   
   revolution axis.  That would probably be the most sensible, if you want   
   the solar power generation to be intrinsic to the habitat itself (rather   
   than, say, beamed from a solar power facility in closer solar orbit).   
      
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