 Galileo published a description of sunspots in 1613 entitled Letters on Sunspots suggesting the Sun and heavens are corruptible . The Letters on Sunspots also reported his 1610 telescopic observations of the full set of phases of Venus , and his discovery of the puzzling " appendages " of Saturn and their even more puzzling subsequent disappearance . In 1615 Galileo prepared a manuscript known as the Letter to the Grand Duchess Christina which was not published in printed form until 1636 . This letter was a revised version of the Letter to Castelli , which was denounced by the Inquisition as an incursion upon theology by advocating Copernicanism both as physically true and as consistent with Scripture . In 1616 , after the order by the inquisition for Galileo not to hold or defend the Copernican position , Galileo wrote the Discourse on the Tides ( Discorso sul flusso e il reflusso del mare ) based on the Copernican earth , in the form of a private letter to Cardinal Orsini . In 1619 , Mario Guiducci , a pupil of Galileo 's , published a lecture written largely by Galileo under the title Discourse on the Comets ( Discorso Delle Comete ) , arguing against the Jesuit interpretation of comets . 
