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Date: 24-2-2016
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Date: 25-8-2020
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Date: 22-7-2020
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The small spherical triangle
When a spherical triangle becomes smaller and smaller on a sphere of fixed radius, its angles remain finite but its sides tend to zero length. The triangle, in fact, approximates more and more to a plane triangle and we would expect that the formulas of spherical trigonometry would degenerate into the well-known formulas of plane trigonometry. This indeed is the case.
Figure 1. flight route from Prestwick to Gander
As an example take the cosine formula, namely cos a = cos b cos c + sin b sin c cos A.
If θ is any angle, in circular measure,
........(1)
If θ is sufficiently small, we may write
cos θ = 1 − θ2/2 sinθ = θ.
Now let the spherical triangle ABC shrink so that the cosine formula may be written as
or
We can neglect b2c2/4 since it is of smaller order than a2/2, etc.
Hence,
a2 = b2 + c2 − 2bc cos A
the well-known formula of plane trigonometry.
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