title: Astropy can't go back and forth between coordinates
body: <p>I have icrs coordinates(SkyCoord) of two sources. Using astropy directional_offset_by() method I have found position angle and separation between them.
  So when I add this position angle and separation to the first Skycoord I should get the second SkyCoord, and I did.
  But when I try to get the first SkyCoord from the second source, I get different results.<br />
  Separation should be the same, but the position angle should be rotated by 180 deg, right?<br />
  I am missing something obvious but can't see what.<br />
  I put below minimum working example.</p>
  <pre><code>Alpheratz = SkyCoord.from_name('Alpheratz') 
  Peg = SkyCoord.from_name(&quot;Psi Pegasi&quot;) 
  Pos_ang = Alpheratz.position_angle(Peg)
  Sep = Alpheratz.separation(Peg)
  print(&quot;Pos_ang:&quot;,Pos_ang,&quot;Sep:&quot;, Sep)
  Peg_from_A = Alpheratz.directional_offset_by(position_angle = Pos_ang, separation = Sep)
  Alphe_back_from_Peg = Peg_from_A.directional_offset_by(position_angle = Pos_ang - 180*u.deg, separation = Sep)
  print(&quot;Peg from name:&quot;,Peg,&quot;\nPeg from A:&quot;, Peg_from_A)
  print(&quot;Alphe from Name:&quot;, Alpheratz, &quot;\nAlphe back from P from A&quot;, Alphe_back_from_Peg)
  </code></pre>