Season start dates: a complex celestial mechanics

What causes explain the change in season dates each year?

The three main causes of variability

The astronomical seasons never begin exactly on the same calendar date. Thus, in the northern hemisphere, spring can begin on March 20 or 21, summer on June 20 or 21, autumn on September 22 or 23, and winter on December 21 or 22. This variability is not arbitrary but results from three main causes: the actual duration of the tropical year (about 365.2422 days), the leap years of the Gregorian calendar (one day added every four years, except for secular exceptions not divisible by 400), and the precession of the equinoxes (a slow movement of the Earth's rotation axis that shifts the seasonal markers by about 20 minutes per century).

The tropical year and the sidereal year

The tropical year

The tropical year is the time it takes for the Earth to return to the same apparent position relative to the Sun, that is, from one spring equinox to the next. It lasts about 365.2422 days, or about 5 hours, 48 minutes and 46 seconds more than a civil year of 365 days. This duration, slightly shorter than the sidereal year (\(365.2564\) days), is due to the precession of the equinoxes, which slowly shifts the seasonal markers over time.

The sidereal year

The sidereal year corresponds to the time it takes for the Earth to complete one full revolution around the Sun relative to the fixed stars, about 365.2564 days. It is slightly longer than the tropical year (365.2422 days) due to the precession of the equinoxes, which makes the seasons drift backward in the calendar. The sidereal year is mainly used in astronomy to track the position of stars in the sky over the years.

Tropical year and civil calendar

An accumulating offset

The tropical year, which measures the time between two successive spring equinoxes, actually lasts about \(\ 365{,}2422 \ \text{days} \). However, our civil calendar is based on years of 365 or 366 days. This difference of ≈0.2422 day accumulates from year to year. Without correction, the equinoxes and solstices would occur later and later in the calendar. Leap years compensate for this delay by adding one day every four years, which makes them slightly advance. A very small residual drift remains: the equinoxes and solstices advance by about one day every 3300 years, thus becoming very slightly earlier over the millennia.

The role of leap years

To compensate for this offset, the Gregorian calendar introduces a leap year every four years, with exceptions: secular years not divisible by 400 are not leap years. For example, the year 1900 was not a leap year (divisible by 100 but not by 400), while the year 2000 was (divisible by 400). This mechanism approximates the duration of the tropical year to \(\ 365{,}2425 \ \text{days} \), reducing the average error to about 26 seconds per year. Despite this, the precise moment of the seasons continues to drift slightly from year to year.

Precession of the equinoxes and other orbital effects

A rotating axis in motion

The Earth does not spin in space like a perfectly stable top. Its rotation axis describes a slow cone, a phenomenon called the precession of the equinoxes. This movement causes a progressive shift of the equinoxes over time (about 20 minutes per century), modifying the seasonal dates over the very long term. For example, the spring equinox, which coincided with the beginning of the sign of Aries 2000 years ago, is now in the constellation of Pisces. This shift, due to precession, very slowly modifies the position of the seasons relative to the stars.

Other orbital irregularities

In addition to precession, there are more subtle effects such as the variation of the eccentricity of Earth's orbit and nutations, which make the dynamics more complex. These phenomena, although minor on a human scale, contribute to the variability of seasonal dates over long periods.

N.B.:
Nutation is a small oscillation of the Earth's axis superimposed on precession, caused by the attraction of the Moon and the Sun on the equatorial bulge. Its main period is about 18.6 years and its effect on seasonal dates remains tiny.

Table of season dates (2025–2040)

The following table presents the precise dates and times of the beginning of the astronomical seasons in Universal Time (UTC) for the period 2025–2040. These data concretely illustrate the variability of dates from one year to the next.