Searching for evidence to support his new understanding of gravitation and his Entwurf Theory, Einstein encounters the problem of the rotation of Mercury’s orbit, which is not explained by Newtonian theory. Together with his friend Besso, he calculates this motion, known as perihelion rotation. Although the result does not match with astronomic observations, he holds fast to the Entwurf Theory. In late 1915, this calculation becomes a prerequisite for the efficient demonstration that his new theory corresponds with astronomic results.