Abstract

We are all used to drawing straight lines to represent time, and above them, we plot historical events or physical or economic data. What to us is a self-evident convention, is however of an astonishingly recent date: it emerged only in the second half of the eighteenth century. To us, this late date seems paradoxical and cries out for an explanation. How else did earlier periods measure change, if not as a function of time? it will be argued that since Antiquity, time was taken to measure change, and change to occur in space. ‘Our’ idea of representing time as an independent dimension would have seemed aberrant. But then, a second issue arises. Did not medieval natural philosophers employ timelines, Oresme’s diagram of the mean speed theorem being the most famous case? However, as will be shown, our interpretation of his diagram is probably wrong. This insight, in turn, takes care of a third paradox, namely Galileo’s initial inability to represent the law of free fall correctly. This article will document that the timeline first emerged in the late sixteenth century in works on chronology, made its first appearance in physics in Galileo’s diagrams, and had its general breakthrough in the eighteenth century.