Twice a year, we move our clocks forward or back an hour. This ritual, now commonplace in our modern lives, is nevertheless the product of a rich history, marked by political decisions, energy crises, and scientific debates. Alongside the "civil" time we use daily, astronomers rely on another scale of measurement: sidereal time. This is based not on the Sun but on the stars and remains essential for understanding and observing the night sky.
THE TUMULTUOUS HISTORY OF DAYLIGHT SAVINGS
The idea of a time difference emerged as early as 1784, when Benjamin Franklin imagined it would be possible to save candles by adapting our days to natural light. But in a society that was still largely agricultural, where life revolved around sunrise and sunset, this suggestion had no effect.
At the end of the 19th century, time standardization became essential. The rise of the railway and the telegraph required a common reference point to avoid confusion and accidents. In 1891, Paris time was adopted as the national time, marking a decisive step in the standardization of time.
The first official switch to summer time took place in Germany in 1916, quickly imitated by other European countries. In France, the idea was adopted the following year. The goal was clear: to save energy by adapting human activities to sunlight.
During the Second World War, the situation became more complicated. The occupied zone adopted German time, while the free zone retained another reference time. After 1945, several decrees attempted to reinstate traditional winter time, but France ultimately remained aligned with Central European Time.
In 1976, following the 1973 oil crisis, President Valéry Giscard d'Estaing officially reinstated daylight saving time. Initially presented as a temporary measure, it became a permanent fixture in French daily life. By the 1980s, all European Union countries had adopted the system to simplify trade. Since then, the clocks have changed twice a year: on the last Sunday of March, clocks are moved forward one hour, and on the last Sunday of October, they are moved back one hour.
HOW DOES THE TIME CHANGE WORK?
In France, the Paris Observatory establishes and disseminates legal time. This is based on atomic clocks, capable of measuring the second with extreme precision, defined since 1967 as the duration of 9,192,631,770 oscillations of the cesium-133 atom. To this reference time, one hour is added in winter and two hours in summer.
In practical terms, this means that in winter, French time is about one hour behind solar time, while in summer, this difference reaches two hours. This compromise allows us to enjoy brighter evenings, but it also distances our days from "natural" solar time.
The primary objective was to save electricity. The savings were indeed measured, but they tend to diminish with the widespread adoption of LED lighting and more energy-efficient equipment. At the same time, critics highlight the health effects, particularly sleep disruptions during the time change, and the impact on certain agricultural practices, such as milking cows, which requires strict regularity.
SET YOUR WATCH PROPERLY DURING THE TIME CHANGE
The switch to summer or winter time is also a delicate moment for watch enthusiasts. Mechanical watches, especially those with a date display, require special care. Between 8 p.m. and 4 a.m., the internal mechanism is running to change the date. This period is known as the "death zone," because handling the watch at this time risks permanently damaging the movement.
It is therefore recommended to adjust the time during the day, or to advance the hands until you are outside this time range before correcting the time and date. Smartwatches and radio-controlled quartz watches generally update automatically, but it is still best to check that the time zone is correctly set.
SIDE TIME: THE HOUR OF THE STARS
Unlike solar time, which is based on the apparent movement of the Sun, sidereal time is based on the Earth's rotation relative to the stars. It is measured using the vernal equinox as a reference point, that is, the intersection of the celestial equator and the ecliptic.
A sidereal day lasts 23 hours, 56 minutes, and 4 seconds, slightly less than a solar day. This difference is explained by the fact that the Earth moves along its orbit around the Sun, which slightly shifts the return of our reference star to the meridian.
There are two forms of sidereal time: true sidereal time, which takes into account the actual position of the equinox, affected by phenomena such as precession and nutation, and mean sidereal time, which is based on a more stable average position over the long term. In practice, the fundamental relationship is as follows: at a given location and time, the sum of a celestial body's right ascension and its hour angle corresponds to true sidereal time. When the celestial body crosses the meridian, its right ascension is equal to the local sidereal time.
Sidereal time is used in astronomy to locate and track celestial objects. Equatorial mounts for telescopes rely on this reference frame to compensate for Earth's rotation and keep a star or galaxy centered in the field of view. It is also essential for compiling star catalogs, planning coordinated observations between multiple observatories, and ensuring the accuracy of certain space missions.
CONCLUSION
Daylight Saving Time illustrates how our societies adapt to energy constraints, international trade, and natural rhythms. While it continues to spark debate and questioning, it is firmly rooted in our European habits.
For its part, sidereal time remains a discreet but fundamental reference point, directly linking our time measurements to the stars. Between social conventions and astronomical constants, these two scales remind us that time, whether civil or cosmic, is above all a construct designed to better situate us in the universe.
Now that you know more about the time change… why not take the opportunity to enrich your collection with a Col&McArthur watch? And so that you can wear it right away, our watchmakers will set it to the correct time before sending it to you.
