The Harvard College Observatory (and the Department of Astronomy)
is located at 60 Garden Street. Garden Street leaves Harvard Square
by Cambridge Commons,   Here is a map
showing the location of the Harvard-Smithsonian Center for Astrophysics
relative to Harvard Square:
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<IMG SRC="http://www.cfa.harvard.edu/~huchra/cfamap.jpg" >
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<IMG SRC="http://www.cfa.harvard.edu/~huchra/perkin.map.sat.jpg" >
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The main entrance to the CfA is up the driveway opposite Linnean Street
(by the Radcliffe Quadrangle and just past the Radcliffe Quad Athletic
Center).   Go up the driveway  and the large 3 story building is
the Perkin Laboratory for Astrophysics.  The Pratt Conference Room
is on the ground floor or Perkin.  Enter the building, the Library
should be on your left.  There is a black doorway just in front of you 
(right opposite the entrance) which leads to a stairwell.
Go down one flight to the G level, turn right
as you leave the stairwell and Pratt is at the end of a short corridor.