title: Angle between two pairs of azimuth and altitude?
body: <p>I have a solar panel pointing (it's normal vector) in some direction. I want to calculate the angle between that and the current position of the sun. I am using pyephem and I have this information in two pairs of azimuth and altitude. </p>
  
  <pre><code>panel_az = ephem.degrees('180')
  panel_alt = ephem.degrees('45')
  sun_az = ephem.degrees('245')
  sun_alt = ephem.degrees('22')
  </code></pre>
  
  <p>What is the easiest way to find the angle between the panel's normal vector and the vector pointing towards the sun?</p>