Spring is on March 20. Why doesn’t the vernal equinox occur on March 21, 2022? We understand why by looking at the position of the Earth and our calendar.
Spring begins on March 20 this year. The spring equinox takes place precisely at 4:33 p.m. this Sunday. But why does the vernal equinox, often associated with the date of March 21, fall a day earlier in 2022?
Since the end of the 16th century, we have used the Gregorian calendar. This solar calendar, divided into 12 months, does not reflect a reality: the Earth takes 365.2422 days (or about 365 days, 5 hours and 48 minutes) to rotate around the Sun. In other words, there is a gap between what is called the tropical year and the calendar year.
The equinox will return on March 21 in… 2102
This shift is compensated by leap years: they consist of adding one more day, every 4 years, at the end of February. This choice has a consequence on the date of the equinox, which is shifted. The March Equinox will no longer fall on March 21 until 2102.
We must also talk about the tilt of the Earth. We talk about the equinox when the Sun is at its zenith on the Earth’s equator, that is to say a point located vertically above our heads. Two equinoxes occur each year: a first between March 19 and 21 (which marks the passage from winter to spring in the northern hemisphere) and a second between September 21 and 24 (the passage from summer in autumn).
Earth’s axis of rotation is tilted
When the Earth revolves around the Sun, its axis of rotation is tilted by approximately 23 degrees, if we take the plane formed by its orbit as a reference. Due to this tilt, the two hemispheres are alternately oriented towards our star. This is the origin of the seasons. For half of the year, the northern hemisphere is more “tilted” towards the Sun, then it is the turn of the southern hemisphere.
It is this situation that changes during an equinox: no hemisphere is oriented more than the other towards the Sun. The star is therefore at the zenith of the equator.
The vernal equinox is not the only one with the shift that we still observe in 2022. It is also valid for the September equinox, which should not fall on the 21st before 2092.