Page "Angle of incidence" Paragraph 8
from
Wikipedia
Determining the grazing angle with respect to a planar surface is trivial, but the computation for almost any other surface is significantly more difficult.
The exact solution for a sphere ( which has important applications in astronomy and computer graphics ) was an open problem for nearly 50 years until a closed-form result was derived by mathematicians Allen R Miller and Emanuel Vegh in 1991.
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