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.
The 1619 arrival of 38 English settlers at Berkeley Hundred in Charles City County, Virginia, concluded with a religious celebration as dictated by the group's charter from the London Company, which specifically required "that the day of our ships arrival at the place assigned ... in the land of Virginia shall be yearly and perpetually kept holy as a day of thanksgiving to Almighty God." The 1621 Plymouth feast and thanksgiving was prompted by a good harvest, which the Pilgrims celebrated with Native Americans, who helped them get through the previous winter by giving them food in that time of scarcity.
In 1619, Galileo became embroiled in a controversy with Father Orazio Grassi, professor of mathematics at the Jesuit Collegio Romano. It began as a dispute over the nature of comets, but by the time Galileo had published The Assayer (Il Saggiatore) in 1623, his last salvo in the dispute, it had become a much wider controversy over the very nature of science itself.