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"The Sidereal Messenger" of Galileo has been for a long time in everybody's hands, also my "Discussion, such as it is, with this Messenger," and my Brief Narrative in confirmation of Galileo's Sidereal Messenger, so any reader may briefly weigh the chief points of that Messenger and see the nature and the value of the discoveries made by the aid of the telescope, the theory of which I am intending to demonstrate in this treatise. Actual sight testified that there is a certain bright heavenly body which we call the Moon. It was demonstrated from the laws of optics that this body is round; also Astronomy, by