When does spring start?

Image description: Astronomically, the equinox occurs at the precise moment when the apparent center of the Sun crosses the celestial equator (yellow). The Earth has an axial tilt of approximately 23.5 degrees (ε) relative to its orbital plane around the Sun (shown in red). The vernal point or point of the spring equinox designates the position of the sun on the celestial sphere at the moment when it passes from the southern celestial hemisphere to the northern celestial hemisphere.
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The Equinox is a precisely determined astronomical moment

The equinox (from the Latin "æquus" equal and "nox" night) is an astronomical event of very short duration. There are two equinoxes each year, in March and September. At this specific time of year (see table below), days and nights last exactly 12 hours.

In the Northern Hemisphere, the start date of spring is also known as the spring equinox. At the poles, the equinox marks the transition between six months of day and six months of night.

This astronomical event is closely linked to the position of the Earth on its orbit in relation to the Sun. The orbital periods of the Earth around the Sun depend on the reference and they are all different.

The orbital period which makes it possible to determine the date of the equinox is the tropical period (tropical year). The tropical year measures, for example, the time elapsed between two successive passages of the Sun at the vernal equinox (beginning of spring). The average length of a tropical year is approximately 365 days, 5 hours, 48 minutes and 45 seconds. This period is relevant for determining the date of the equinox.

From an astronomical point of view, the equinox occurs at the precise moment when the apparent center of the Sun crosses the celestial equator (see image).
This precise point is called the vernal point or the equinoctial point. At this time, the length of day is approximately equal to the length of night over the entire surface of the Earth.

The equinoxes occur between March 19 and 21 (beginning of spring in the northern hemisphere), and between September 21 and 24 (beginning of autumn in the northern hemisphere).

Dates of the last 10 spring and autumn equinoxes

The calendar year is 365 days long and the tropical year is approximately 365 days, 5 hours, 48 minutes and 45 seconds.

The equinoxes therefore occur approximately six hours later, from one year to the next. This small fraction of an extra day is why we have leap years every four years, to compensate for the excess time accumulated and keep the seasons aligned with the calendar.

Leap years shift the equinox dates by one day every four years. Approximately every 130 years, the equinoxes may occur a day earlier due to the gradual accumulation of this difference of 5 hours, 48 minutes, and 45 seconds each year, despite leap adjustments. This is partially compensated by the absence of a leap year for years divisible by 100 (but not by 400). Thus the alignment of the seasons, with the long-term calendar, is maintained.

This synchronization, which only occurs every four years, explains why the date of the start of spring varies over 3 days between March 19 and 21.

When does spring start?

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