 Because The Assayer contains such a wealth of Galileo 's ideas on how science should be practised , it has been referred to as his scientific manifesto . Early in 1619 , Father Grassi had anonymously published a pamphlet , An Astronomical Disputation on the Three Comets of the Year 1618 , which discussed the nature of a comet that had appeared late in November of the previous year . Grassi concluded that the comet was a fiery body which had moved along a segment of a great circle at a constant distance from the earth , and since it moved in the sky more slowly than the moon , it must be farther away than the moon . 
