The tennis racket theorem, or intermediate axis theorem, is a kinetic phenomenon of classical mechanics which describes the movement of a rigid body with three distinct principal moments of inertia. It has also been dubbed the Dzhanibekov effect, after Soviet cosmonaut Vladimir Dzhanibekov, who noticed one of the theorem's logical consequences whilst in space in 1985. The effect was known for at least 150 years prior, having been described by Louis Poinsot in 1834 and included in standard physics textbooks such as Classical Mechanics by Herbert Goldstein throughout the 20th century.