title: pyephem conversion of (Alt, Az) to (Ra, Dec) and back not internally consistent
body: <p>I am finding that when I convert an (Alt, Az) to an (Ra, Dec) and then back with PyEphem, I don't get what I started with.  Below is a simple example.</p>
  
  <pre><code>import ephem
  print ephem.__version__
  # '3.7.3.4'
  
  gbt = ephem.Observer()
  gbt.long = '-79:50:23.4'
  gbt.lat = '38:25:59.23'
  gbt.pressure = 0 # no refraction correction.
  gbt.epoch = ephem.J2000
  # Set the date to the epoch so there is nothing changing.
  gbt.date = '2000/01/01 12:00:00'
  
  # Should get the north pole right?
  ra, dec = gbt.radec_of(0, '38:25:59.23')
  # Not the north pole... error might be abberation.
  print dec
  # 89:59:30.5
  
  # Now check internal consistancy by reversing the calculation.
  pole = ephem.FixedBody()
  pole._ra = ra
  pole._dec = dec
  pole._epoch = ephem.J2000
  pole.compute(gbt)
  # Should get what I started with right?
  alt = pole.alt
  # Not what I started with... error unknown.
  print alt
  # 38:26:26.7
  </code></pre>
  
  <p>As noted in the comments, not getting exactly the north pole might just be stellar aberration, although the 30" is more than Wikipedia stated maximum effect of 20".</p>
  
  <p>The fact that I don't get the same thing when I do the reverse calculation is truly puzzling me.  Any suggestions?</p>