Libra Astronomica
- Title page.
- Orazio Grassi (1583-1654).
- Libra astronomica ac philosophica qua Galilaei Galilaei opiniones de cometis a Mario Guiducio in Florentina Academia expositae, atque in lucem nuper editae, examinantur a Lothario Sarsio Sigensano.
- Perugia: Marco Naccarini, 1619.
In that same year of 1619, Grassi responded to Galileo with the publication of a treatise under the pseudonym of Lothario Sarsi of Siguenza: Libra astronomica ac philosophica qua Galilaei Galilaei opiniones de cometis a Mario Guiducio in Florentina Academia expositae, atque in lucem nuper editae, examinantur a Lothario Sarsio Sigensano (Astronomical and Philosophical Weighing Scales, in which Galileo Galilei’s Opinions about Comets, explained by Mario Guiducci in the Florentine Academy, and recently published, are examined by Lotario Sarsi of Siguenza). The title page includes an engraving depicting the last of the three comets as it was appearing in the constellation of Libra, the weighing scales, highlighting the metaphorical meaning of the title: the theories of Galileo and Grassi should be judged by the weight of their respective arguments.