Thales of Miletus is recognized for popularizing geometry in ancient Greek culture and is credited with separating trigonometry as an independent discipline from mathematics. The foundational work on trigonometric functions, particularly the length of chords in a circle, was pioneered by Hipparchus around 140 BC, who created the first known table of chords. This historical context establishes Hipparchus as the true founder of trigonometry, while Thales contributed significantly to the geometric principles that underpin trigonometric concepts.
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http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Trigonometric_functions.htmlThe first work on trigonometric functions related to chords of a circle. Given a circle of fixed radius, 60 units were often used in early calculations, then the problem was to find the length of the chord subtended by a given angle. For a circle of unit radius the length of the chord subtended by the angle x was 2sin (x/2). The first known table of chords was produced by the Greek mathematician Hipparchus in about 140 BC. Although these tables have not survived, it is claimed that twelve books of tables of chords were written by Hipparchus. This makes Hipparchus the founder of trigonometry.