Solstice: why our watches mislead us about the length of the day

Our watches mislead us about the true length of a day and the precise moment when the Sun reaches its highest point. The effect is especially noticeable at the solstices, including yesterday.

The summer solstice and the length of the day

In the Northern Hemisphere, 21 June is the year’s longest day: the summer solstice. The term comes from solstitium, built from the Latin roots sol (“Sun”) and stitium (derived from the verb stare, meaning “to stop”, “to become still” or “to stand motionless”). Literally, solstice therefore means “the Sun stops” or “the stopping of the Sun”. Yet this etymology does not explain why this day has so many hours of sunshine, nor, more importantly, “for whom is it longer?” To understand it properly, we need to draw on other disciplines.

Brittany and Brasília: a misleading sunset gap

Yesterday, in Brasília, Brazil’s capital, the Sun set at about 17:12. In Brittany, meanwhile, its bright rays lingered until shortly before 22:00 - almost five hours later in the day. At first glance, there is nothing surprising about this: Brittany lies in the Northern Hemisphere, which is tilted towards the Sun at this time of year, so its residents enjoy exceptionally long days. Brazil, by contrast, is in the Southern Hemisphere: for Brazil, it is the winter solstice, the shortest day of the year.

However, Brasília is only 16° from the Equator, where the difference in daylight duration between the longest and shortest days of the year does not exceed one and a half hours. So how can the apparent gap be five hours?

The culprits: time zones

Although this is an astronomical phenomenon, the issue must also be examined through politics and geography. At the International Meridian Conference in Washington in 1884, 25 nations adopted the principle of standardised time zones based on the Greenwich meridian. That event established the foundations of the shared UTC standard, Coordinated Universal Time, which remains in use today.

Metropolitan France, for instance, officially operates on UTC+2 during summer: its clocks display two hours more than the universal reference time. Brazil, on the other hand, spans four time zones, from UTC-2 to UTC-5, with Brasília located in UTC-3.

Converting sunset times to UTC

To make sense of yesterday’s Brittany–Brasília sunset paradox, convert both times into their UTC value. Brasília’s local sunset at 17:12 was 20:12 UTC. Brest’s sunset, shown as 21:47 in French time (UTC+2), was therefore 19:47 UTC. In absolute real time, the Brazilian Sun set 25 minutes after the one in Brittany - an outcome that makes sense when compared with the difference in daylight duration between the two countries at the solstice.

In reality, our watches and smartphones “invent” those five hours: France moves two hours ahead of UTC, while Brazil falls three hours behind it. These time conventions add five hours to a genuine solar difference of 25 minutes.

You can repeat this simple exercise at home: select two far-apart cities, such as Tokyo (UTC+9) and Mexico City (UTC-6). Find their sunset times on any solar ephemeris website - timeanddate.com, sunrise-sunset.org, or suncalc.org for map enthusiasts - then subtract nine hours from Tokyo’s sunset time, add six hours to Mexico City’s, and compare both UTC results. The 15 hours that artificially separate their clocks vanish as soon as the mask of time zones is removed.

The extra second

UTC itself is now being questioned because it depends too heavily on Earth’s rotation, which is inherently irregular. To compensate for this, the leap second was introduced in 1972, allowing UTC to remain aligned with Earth by occasionally adding one second to atomic clocks.

Since then, Google, Meta, Amazon and global financial servers have regarded it as their nemesis: adding a single second every two or three years can be enough to crash computer systems, desynchronise satellites, halt stock-market transactions or disrupt airlines. They consequently won their case with the International Bureau of Weights and Measures (BIPM) in 2022, which formally approved the end of leap seconds by 2035 at the latest. You will not notice any difference: the gap between your watch and the Sun will grow by no more than one minute per century, while the “solstice paradox” - which is not really a paradox - will remain unchanged.