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‘It’s natural,’ says the Stanford Encyclopedia of Philosophy, ‘to think that time can be represented by a line.’ We imagine the past stretching in a line behind us, the future stretching in an unseen line ahead. We ride an ever-moving arrow – the present. However, this picture of time is not natural. Its roots stretch only to the 18th century, yet this notion has now entrenched itself so deeply in Western thought that it’s difficult to imagine time as anything else. And this new representation of time has affected all kinds of things, from our understanding of history to time travel.

Let’s journey back to ancient Greece. Amid rolls of papyrus and purplish figs, philosophers like Plato looked up into the night. His creation myth, Timaeus, connected time with the movements of celestial bodies. The god ‘brought into being’ the sun, moon and other stars, for the ‘begetting of time’. They trace circles in the sky, creating days, months, years. The ‘wanderings’ of other, ‘bewilderingly numerous’ celestial bodies also make time. When all their wanderings are ‘completed together’, they achieve ‘consummation’ in a ‘perfect year’. At the end of this ‘Great Year’, all the heavenly bodies will have completed their cycles, returning to where they started. Taking millennia, this will complete one cycle of the universe. As ancient Greek philosophy spread through Europe, these ideas of time spread too. For instance, Greek and Roman Stoics connected time with their doctrine of ‘Eternal Recurrence’: the universe undergoes infinite cycles, ending and restarting in fire.

Such views of time are cyclical: time comprises a repeating cycle, as events occur, pass, and occur again. They echo processes in nature. Day and night. Summer to winter. As the historian Stephen Jay Gould explains in Time’s Arrow, Time’s Cycle (1987), within the West, cyclical conceptions dominated ancient thought. It’s even hinted at in the Bible. For example, Ecclesiastes proclaims: ‘What has been will be again … there is nothing new under the sun.’ Yet, Gould writes, the Bible also contains a linear conception of time: time comprises a one-way sequence of unrepeatable events. Take Biblical history: ‘God creates the earth once, instructs Noah to ride out a unique flood in a singular ark.’ Gould describes this linear understanding of history as an ‘important and distinctive’ contribution of Jewish thought. Biblical history helped power linear ideas of time.

Cyclical and linear conceptions of time thrived side by side for centuries, sometimes blurring into one another. After all, we live through natural, cyclical seasons and unrepeatable events – birth, first marriage, death. Importantly, medievals and early moderns didn’t literally see cyclical time as a circle, or linear time as a line. Yet in the 19th-century world of frock coats, petticoats and suet puddings, change was afoot. Gradually, the linear model of time gained ground, and thinkers literally began drawing time as a line. To explain how, I point to four key developments.

First: chronography, the art of representing historical events. Historians have always struggled with how to best display events on a page, and since ancient times a popular solution lay in ‘time tables’: grids displaying dates.

From the 1856 edition of Blair’s Chronology. Public domain. Courtesy the Internet Archive

A crucial innovation lay in the invention of ‘timelines’. As Daniel Rosenberg and Anthony Grafton detail in their coffee-table gorgeous Cartographies of Time (2010), the ‘modern form’ of the timeline, ‘with a single axis and a regular, measured distribution of dates’, came into existence around the mid-18th century. In 1765, the scientist-philosopher Joseph Priestley, best known for co-discovering oxygen, invented what was arguably the world’s first modern timeline.

From A Chart of Biography (1765) by Joseph Priestley. Courtesy the National Endowmment for the Humanities

On his A Chart of Biography (1765), time flows from left to right, from 1200 BCE to........

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