One of those unnoticed monographs that changed the world. It is an account of the methods used to observe, by telescope, the transit of Mercury across the sun as predicted by Kepler - using his Rudolphine tables combined with his heliocentric theory. The method was to project the sun's image onto a screen and use the help of an assistant to time the entry and exit points. Gassendi noted the relatively infinitesimal size of the planet. The observation confirmed that Mercury's orbit was approximatively elliptical with the sun at one of the foci. Conducted just two years before the trial of Galileo this was a critical experiment. All those who had read and understood this paper - including some Jesuits like Honore de Fabre - knew very well that the Pope and the Inquisition had placed the Church in an untenable position. Later observations, conducted by Gassendi and Fr Bouilliau, enabled them to confirm that Mercury was actually the most elliptical of all the planets. Newton later used these figures in his Principia. Being a man of discretion Gassendi kept his confirmed heliocentricity secret to the grave.