Ω

θ

Figure N.1: Relation between plane angleand solid anglefor a right circular cone.

There is a simple relation between plane angle and solid angle for a right circular cone. If the vertex of
the cone subtends an angle(theaperture angleof the cone), then the corresponding solid angleis (Fig.
N.1)

D2



1 cos



2



: (N.3)

Another unit of solid angle is thesquare degree(deg^2 ):

sq:deg:Dsr



180



 2

: (N.4)

In these units, a hemisphere is 20,626.48 deg^2 , and a complete sphere is 41,252.96 deg^2.