VA Vertex Angle - the angle of the event around the limb of the moon measured anticlockwise from the vertex 
of the lunar limb - i.e. the point on the limb highest from the horizon 

AA Axis Angle - the angle of the event around the limb of the moon, measured eastward from the moon's north pole. 
Essential for reappearance, as it locates the event with reference to lunar features. To use, mark a map of the 
moon around the circumference at 10 deg intervals, starting at the north pole. Mare Crisium is at about 300 deg. 

[Note - the so-called Watts Angle differs from the Axis Angle by 0.21 deg. The term Watts Angle is limited to the 
context of accessing the Watts Charts. In all other situations, the term Axis Angle is preferred.]