In 2023, the summer solstice corresponds to Wednesday, June 21 in the northern hemisphere. But, this is not the case every year. Sometimes the event falls the day before, June 20. Why doesn’t the summer solstice always take place on June 21?
Summer is coming, it’s official — preceded in recent days by already summery heat and unpredictable thunderstorms. In the northern hemisphere, the day of the summer solstice falls on Wednesday, June 21, 2023. This astronomical event takes place precisely at 4:57 p.m. Very often, the date of June 21 is associated with the first day of summer, and this is the case this year. However, this is not set in stone, even if our calendar is designed to stay close to a fixed date for the start of the different seasons. Why doesn’t summer always fall on the same date?
To understand it, we must first remember what happens in the sky during the June solstice. This term designates the moment when the northern hemisphere of the Earth is directed towards the Sun (during the winter solstice, it is the southern hemisphere). The following diagram helps to better understand the position of our planet, which is itself tilted, with respect to the Sun.
During the summer solstice, the Sun passes through the zenith of the Tropic of Cancer (Capricorn for the winter solstice). The zenith designates the imaginary point that is vertically above our heads, 90° from any point on the horizon. Be careful, the zenith does not designate the highest point of a star that travels a trajectory in the sky: to say that the Sun reaches its zenith when it is high in the sky is an abuse of language. In this case, we speak of a climax.
Why is a shift possible?
Now back to the calendar. As the Institute of Celestial Mechanics and Ephemeris Calculation of the Paris Observatory explains, the dates of the summer solstices do not necessarily fall on June 21. The institute has listed the dates of the summer solstice from 1583 to 2999: some fall on June 20, such as in 2016 or 2020. In the future, the institute even anticipates that the event will take place on June 19 (but not before 2488). The summer solstice fell on June 22 in 1975 and it won’t happen again until 2203.
Why this discrepancy? It comes from our Gregorian calendar, officially introduced in 1582. This calendar introduced a leap year every 4 years. Indeed, a calendar year lasts 365 days, which does not quite correspond to the time it takes for the Earth to make one revolution around the Sun (ie 365 days, 5 hours and 48 minutes). Leap years attempt to correct the discrepancy between this time of revolution and the adopted calendar.
On June 21, 2023, the northern hemisphere will officially enter summer. Seen from Paris, the Sun will rise at 5:49 a.m. and set at 9:56 p.m.
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