Galileo's planetary observations from 'Il Saggiatore' (1623, 'The Assayer'). This work by Italian astronomer Galileo Galilei (1564-1642), was about comets and contained treatises on natural philosophy and mathematics. The illustration on this page shows Saturn (centre left) and its rings. At centre right are Jupiter and Mars. Across bottom are observed phases of Venus. Galileo's observations with his telescope challenged the existing worldview that said the heavens were perfect and unchanging.