This Saturday, June 21, 2025 marks the start of the summer season. Discover all the explanations on this astronomical phenomenon.
The summer of 2025 started very precisely this Saturday, June 21, at 4:42 am according to the figures provided by theInstitute of celestial mechanics and calculation of ephemeris (IMCCE), due to the astronomical phenomenon of the summer solstice … But what is it? Vast question! To put it simply, this difficult event to pronounce is actually an astronomical episode which takes place twice a year, six months apart. It occurs when the position of the sun reaches its maximum distance from the equator.
The summer solstice, which takes place each year between June 20 and 22 in our calendar, corresponds to the longest day of the year in the northern hemisphere … Conversely of the winter solstice, between December 20 and 22, which designates, still in our hemisphere, the shortest day of the year.
How to define the summer solstice? It corresponds to the time of the year when the trajectory of the sun, seen from the earth, reaches its northern (in the northern hemisphere) or southern (in the southern hemisphere). In other words, during the summer solstice, at our latitudes, the sun “passes” to its zenith, at the level of the tropic of cancer. In reality, it is not a movement of our star that produces this phenomenon, but the earth revolution around the Sun. Indeed, the axis of our planet is tilted by 23 degrees compared to the plan of theeclipticthat is to say the plan on which the earth moves in relation to the sun. We therefore have the illusion of a displacement of the sun in the sky. But a good diagram is sometimes better than a long speech …
Concretely, this phenomenon has many consequences that can be easily seen in the naked eye (and with a clock in hand). It is at the time of the solstice that the sun passed the highest in the sky at the moment of its zenith : The shadows are shortened to the maximum. The star can then go, for example, to 65 ° altitude compared to the horizon. By way of comparison, at the time of winter solsticethe sun is only 18 ° compared to the horizon. Another consequence: it is at this moment that the star gets up and sets the most towards the north (northeast in the morning, north-west in the evening).
The most obvious consequence of the phenomenon of the summer solstice is the maximum extension of the duration of the visible day. From the next day, the duration of the day will begin to go down gradually. First imperceptibly, then faster and faster to the fall equinox. Latin etymology of the word solstice also translates this progression: Sol (sun) and sistere (stop) recall that it was during this period that the duration of the day evolves the least quickly.
If the duration of the day reaches its maximum on the occasion of the solstice, it is not the same everywhere. The more we approach the Arctic Polar Circle, the more it increases. In the north of Norway, the sun hardly descends under the horizon: there is no night per se. There, the sun stops lying down 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 regions, Antarctica is plunged into darkness. Be careful however: if it is theoretically at the time of the summer solstice that the sun rises the earliest in the morning or sets the latest at night, this principle is not always rigorously exact. In question, the slightly elliptical race of the earth around the sun, marked by accelerations and slowdowns. The irregular form of our planet also plays a role.
This is simply explained by the Gregorian calendar, as explained by the site Numerama. The latter was adopted in 1582 and it plans that a year lasts 365 days, except for the bissextile years which take place 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 a revolution around the sun, it takes precisely 365 days, 5 hours and 48 minutes. The decision to include bissextile years in the calendar has been taken to erase the gap between the duration of a year and the time of revolution. And this explains the shift in summer solstices.
While the phenomenon also falls on the day of the music festival, the summer solstice traditionally gives rise to the great feast of fire in Saint John, which is then celebrated every June 24. This celebration honors the summer season inspired by the cult of the sun during Antiquity. A first pagan practice of celebration of the harvests taken up by the Catholic Church, which Christianized it to erect it as a feast of light, in particular thanks to the famous fires of Saint-Jean lit at nightfall.
In our northern hemisphere, the summer solstice corresponds to the longest days of the year. The time that separates sunrise and sunset can exceed 16 hours, against just 8 hours at the end of December at the time of winter solstice. The phenomenon stems from the natural inclination of the earth in relation to the plane on which it revolves around the sun. Without this discrepancy, the seasons as we know them on our planet would not exist! In our regions, the summer solstice therefore sounds the start of the summer season. Halfway through spring and autumn equinoxeswhen night and day have exactly the same duration. What make it a very special date.
The date of the summer solstice is generally the first information that we are trying to know among gardeners, lovers of the hottest days or simply in those who like to be fixed on the seasons. On this point, it is relatively simple: The summer solstice generally took place on June 21. It also arrived in 2008, 2012 or 2020, that the summer solstice intervened 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 is evolving each year. It is a question of properly assessing the trajectory of the earth compared to the sun, and our ability to calculate our hourly system and our calendar on these astronomical movements. Indeed, calendar year (365 days) does not correspond to the tropical yearthat is to say to the exact duration that our star is turning around our star (365 days, 5 hours, 48 minutes and 45 seconds). If this “discrepancy” is corrected by Bissextile yearsthe elliptical trajectory of the earth also plays a role which shifts the solstice each year.
Here is what dates will take place the summer solstices of the following years:
- Summer solstice 2025: June 21
- Summer solstice 2026: June 21
- Summer solstice 2027: June 21