SUMMER SOLSTICE. The first day of summer 2024 takes place this Thursday, June 20. At what time does the astronomical event occur and why not on June 21 as usual? Explanations.
The start of summer begins this Thursday, June 20, 2024, the day when sunlight lasts the longest in the day. The summer solstice actually corresponds to an astrological phenomenon: it is the moment when the Sun “passes” at its zenith, at the Tropic of Cancer. The phenomenon will take place at 8:50 p.m. UTC, i.e. has 10:50 p.m. French time. From this time of year, the days become shorter and shorter until winter and its solstice which takes place on December 21.
After the summer solstice, the day of June 21 will be just as long. It is therefore from June 22 that the duration of sunlight will decline to arrive at the shortest day, the winter solstice, which corresponds to approximately eight hours of light. This year, the summer solstice will not take place on June 21 as usual but on the 20th. As the site explains Science and Future, during the 21st century, this event will occur 47 times on June 20. This solstice happened 36 times during the 20th century on June 22 and 64 times on June 21.
How to define the summer solstice? It corresponds to the time of year when the trajectory of the Sun, seen from Earth, reaches its northern (in the northern hemisphere) or southern (in the southern hemisphere) maximum. In other words, during the summer solstice, at our latitudes, the Sun “passes” at its zenith, at the Tropic of Cancer. In reality, it is not a movement of our star which produces this phenomenon, but rather the revolution of the Earth around the Sun. Indeed, the axis of our planet is inclined by 23 degrees relative to the plane of theecliptic, that is to say the plane on which the Earth moves relative to the sun. We therefore have the illusion of a movement of the Sun in the sky. But a good diagram is sometimes better than a long speech…
Concretely, this phenomenon has numerous consequences that can be easily seen with the naked eye (and with a clock in hand). It is at the time of the solstice that the sun passes highest in the sky at its zenith. : the shadows are shortened as much as possible. The star can then pass, for example, at an altitude of 65° relative to the horizon. For comparison, at the time of winter solstice, the sun only rises 18° from the horizon. Another consequence: this is when the star rises and sets most towards the north (northeast in the morning, northwest in the evening).
The most obvious consequence of the summer solstice phenomenon is the maximum lengthening of the length of the visible day. From the next day, the length of the day will begin to gradually decrease. At first imperceptibly, then more and more quickly until the autumnal equinox. The Latin etymology of the word solstice also translates this progression: sol (sun) and sistere (to stop) point out that it is during this period that the length of the day changes the least rapidly.
If the length of the day reaches its maximum on the occasion of the solstice, it is not the same everywhere. The closer we get to the Arctic Circle, the more it increases. In the north of Norway, the Sun hardly goes below the horizon: there is no night to speak of. There, the Sun completely stops setting from the summer solstice at the Arctic Circle. In the southern hemisphere, the phenomenon is reversed. At the time of the summer solstice in our region, Antarctica is plunged into darkness. Be careful, however: if it is theoretically at the time of the summer solstice that the sun rises earliest in the morning or sets latest in the evening, this principle is not always strictly accurate. The cause is the slightly elliptical path of the Earth around the Sun, marked by accelerations and slowdowns. The irregular shape of our planet also plays a role.
This is simply explained by the Gregorian calendar, as explained on the site Numerama. The latter was adopted in 1582 and it provides that a year lasts 365 days, except for leap years which occur every four years and during which there are 366 days with the addition of February 29. In fact, more than 365 days are necessary for the Earth to make one revolution around the Sun, it takes precisely 365 days, 5 hours and 48 minutes. The decision to include leap years in the calendar was made to erase the gap between the length of a year and the time of revolution. And this explains the shift in the summer solstices.
While the phenomenon also falls on the music festival, the summer solstice traditionally gives rise to the great fire festival of Saint John, which is then celebrated every June 24. This festival honors the summer season by drawing inspiration from the cult of the sun during Antiquity. An initially pagan practice of celebrating the harvests taken up by the Catholic Church, which Christianized it to establish it as a festival of light, in particular thanks to the famous Saint John fires lit at nightfall.
In our Northern Hemisphere, the summer solstice corresponds to the longest days of the year. The time between sunrise and sunset can exceed 16 hours, compared to just 8 hours at the end of December at the time of winter solstice. The phenomenon arises from the natural inclination of the Earth relative to the plane on which it revolves around the Sun. Without this gap, the seasons as we know them on our planet would not exist! In our regions, the summer solstice therefore marks the start of the summer season. Halfway between the vernal equinoxes andautumn, when night and day have exactly the same duration. Enough to make it a very special date.
The date of the summer solstice is generally the first information that we seek to know among gardeners, lovers of the hottest days or simply among those who like to be fixed on the ball of the seasons. On this point, it’s relatively simple: The summer solstice usually occurs on June 21. But be careful, there are still exceptions like this year 2024. It also happened in 2008, 2012 or 2020 that the summer solstice occurred on June 20.
This phenomenon can also occur on June 22, as was the case in 1975 (the next occurrence will take place in… 2203). Even rarer, the summer solstice can finally arrive on June 19. It will be in 2488. Because the exact moment of the solstice changes every year. It is a question of properly evaluating the trajectory of the Earth in relation to the Sun, and of our ability to model our time system and our calendar on these astronomical movements. Indeed, the calendar year (365 days) does not correspond to the tropical year, that is to say the exact duration it takes for our body to revolve around our star (365 days, 5 hours, 48 minutes and 45 seconds). If this “shift” is corrected by the leap yearsthe Earth’s elliptical trajectory also plays a role in shifting the solstice each year.
Here are the dates on which the summer solstices will take place in the following years:
- Summer solstice 2025: June 21
- Summer solstice 2026: June 21
- Summer Solstice 2027: June 21