n the basis of these observations of space and time, the first astronomers divided time into ‘compartments’, purely conventional ‘slices’ that became the 24 hours of world time, each hour divided into 60 minutes, each minute divided again into 60 seconds.

As long as people remained sedentary or travelled on foot, or even on horseback, they quite naturally based their readings of time on the ‘true’ local hour encountered along the way, as determined by the midday sun and observation of the heavens. With the invention of mechanical watchmaking, greater priority was given to an artificial ‘mean time’ – ‘equinoctial time’ as established on the basis of the mean duration of the solar day, even though, as we know, the duration of daytime and night-time is equal only at the spring and autumn equinoxes.

As Lucien Baillaud, the author of the study ‘Les chemins de fer et l’heure légale’ writes: “You could not expect watchmakers to build clocks with speeds that varied according to the time of year.” That is all very well. But these local times, however rational they were locally, varied according to longitude and were useful only to the sedentary. As increasingly faster transport and communications developed, the accumulation of different local times along the same longitude became cumbersome.

"People had to become aware of the inconvenience of local times, invent a time system of broader geographical value, pursue a national, then international, plan for introducing a single time system, develop a ‘standard’ time, find the practical means for applying this single time system, convince the key people about the opportuneness of all this and then executing it.” A vast programme, both national, political and international, that Dominique Fléchon recounts on the following pages.

The international standardisation of time across the globe, now divided into ‘time zones’ (including a number of geopolitical aberrations), opened up new territory for watchmakers to explore: how to show the times of the whole world with one single mechanism? Or at least two di erent times: ‘away’ and ‘home’?

As we will see in this portfolio devoted to travel and time, various solutions were found. But they are all hallmarks of a period that the development of electronics and, today, smartwatches, have swept away: a period when watchmaking ruled and timepieces were truly indispensable to travellers – whether by road, rail, sea and then air – and to anyone wanting to wire, then phone or telex to the other side of the world.

Today, consulting your smartphone is certainly much simpler. But the beauty and mechanical ingenuity of these horological items, and in particular world time watches, still enchant us. By offering us an immediate, summarised, graphical view of all the times on our planet suspended in the cosmos, these watches provide a link to the mystery of our existence, so closely dependent on this alternation of day and night.