This applet gives a demonstration of the formula for the area of a spherical triangle. We can choose the points P, Q, and R on the sphere and display the triangle they determine. By entering "false" in the Boolean expressions, we can shut off the two lunes of a given color which start at a vertex of the triangle, leaving only the triangle and its antipodal triangle. The area of the double-lune with angle α is to 4π as α is to 2π so the double-lune area is 4α. The three double-lunes intersect exactly in the triangle T and its congruent antipodal triangle, covering the rest of the sphere exactly once. Thus the sum of the areas of the double lunes will be 4π + 4A(T) = 4α + 4β + 4γ so A(T) = α + β + γ - π. This is the famous formula of Herriot and Giraud.