Merge pull request #267 from sblunt/faq_docs

add faq on inc, aop, pan

docs/faq.rst

@@ -6,6 +6,12 @@ Frequently Asked Questions
 Here are some questions we get often. Please suggest more by raising an 
 issue in the `Github Issue Tracker <https://github.com/sblunt/orbitize/issues>`_.
 
+**What does this orbital parameter mean?**
+
+We think the best way to understand the orbital parameters is to see how they affect the orbit visually.
+Play around with this `interactive orbital elements notebook <https://github.com/sblunt/orbitize/blob/main/docs/tutorials/show-me-the-orbit.ipynb>`_ 
+(you'll need to run on your machine).
+
 **What is τ and how is it related to epoch of periastron?**
 
 We use τ to define the epoch of periastron as we do not know when periastron will be for many of our directly
@@ -17,5 +23,17 @@ imaged planets. A detailed description of how τ is related to other quantities
    faq/Time_Of_Periastron.ipynb
 
 
+**Why is the default prior on inclination a sine prior?**
+
+Our goal with the default prior is to have an isotropic distribution of the orbital plane.
+To obtain this, we use inclination and position angle of the ascending node (PAN) to
+define the orbital plane. They actually coorespond to the two angles in a spherical coordinate system:
+inclinaion is the polar angle and PAN is the azimuthal angle. Becuase of this choice of coordinates,
+there are less orbital configurations near the poles (when inclination is near 0 or 180), so we use
+a sine prior to downweigh those areas to give us an isotropic distribution. 
+A more detailed discussion of this is here:
 
+.. toctree::
+   :maxdepth: 1
 
+   faq/Orientation_Of_Orbit.ipynb