Henry Perigal, Jr. FRAS MRI (1 April 1801 – 6 June 1898) was a British stockbroker and amateur mathematician, known for his dissection-based proof of the Pythagorean theorem and for his unorthodox belief that the moon does not rotate.

[1][2][3][4][5][6] Perigal descended from a Huguenot family who emigrated to England in the late 17th century,[4][5][6] and was the oldest of six siblings.

[6] He attended the Royal Institution regularly as a visitor for many years, and finally became a member in 1895, at age 94.

[5] In his booklet Geometric Dissections and Transpositions (London: Bell & Sons, 1891) Perigal provided a proof of the Pythagorean theorem based on the idea of dissecting two smaller squares into a larger square.

He falsely believed that the moon does not rotate with respect to the fixed stars, and used his knowledge of curvilinear motion in an attempt to demonstrate this belief to others.