The concept of a spherical Earth dates back to around the 6th century BC, in ancient Indian philosophy, and in ancient Mediterranean thought. It remained a matter of philosophical speculation until the 3rd century BC when Hellenistic astronomy established the spherical shape of the earth as a physical given. A practical demonstration of Earth's sphericity was achieved by Ferdinand Magellan and Juan Sebastián Elcano's expedition's circumnavigation (1519−1521).