Trigonometry
is a branch of mathematics that means "measurement of, with and by means of triangles".

Astronomers as early as 150 BC developed the study of trigonometry. By about 1500 AD
the trig. ratios (sine, cosine and tangent) were established. Once again,
right-angled triangles, as in Pythagoras' Theorem, are important when making
calculations with the trig ratios.