Mapping the Universe presents a multi-disciplinary and multi-author exploration of Robert Grosseteste's treatise On the Sphere. Composed probably in the second decade of the thirteenth century, On the Sphere is part of a series of works on natural phenomena Grosseteste wrote between c.1195 and c.1230. In this work the focus is on astronomy and geometry. Grosseteste moves through issues on the sphericity of the Earth, the movements of celestial bodies, especially the Sun and Moon (including their eclipses), how Earth's climate is affected by these bodies, and how time and the seasons are measured. Drawing on a wide range of sources from the ancient world translated into Latin from Greek and Arabic, Grosseteste's On the Sphere is based, fundamentally, on a discussion of not just how astronomy operates but why. Aristotle and Euclid are the principal framework for his reasoning and presentation. This volume includes an English translation of the treatise, an online celestial model to consult alongside the text, commentary on its striking aspects such as precession and epicycles, discussion of the historical and intellectual context including Grosseteste's concurrent interests in pastoral care, and its illuminated versions. Presented by a team of medievalists and modern scientists, the volume uses this multi-disciplinary methodology to elucidate the treatise and showcase its originality.