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I, like most #Irish people, learned in #school that the seasons start on the first days of February, May, August, and November. This is reflected in the Irish names of September and October - they are Mean Fómhair and Deireadh Fómhair, which literally translate as Mid-Autumn and Late Autumn.

But then when I got the Internet, I kept seeing #Google make a big song and dance over the new season about six weeks after the season started. And Yanks in fora apparently believed them.

Turns out there are a few definitions of the seasons, and Met Éireann doesn't even use the traditional Irish system. According to Met Éireann, Autumn starts in the month called "Mid-Autumn" in Irish, and the month called "Late Autumn" is the middle of autumn.

This is even more confusing for people in the southern hemisphere, for whom seasons are inverted compared the the northern hemisphere.

Ireland, and most of Europe for the bulk of history, used the insolation definition of the seasons - #Summer is the brightest three months of the year, #Winter is the darkest three months, and #Spring and #Autumn come between. Seasons begin on (approximately) the cross-quarter days, and the solstices and equinoxes fall (approximately) halfway through the seasons.

This is why #Shakespeare's *A Midsummer Night's Dream* takes place on the summer solstice, which is midsummer.

A more astronomically rigorous version of this system is used in China, Mongolia, Korea, Japan, and Vietnam. Spring officially begins on the day that the sun crosses 315° of the ecliptic, Summer when it crosses 45°, Autumn when it crosses 135°, and Winter when it crosses 225°. Solstices and equinoxes fall at the exact halfway points of the seasons.

Seasons are thus completely decoupled from the legal months and the phases of the moon.

The seasonal definition used in most of the world is the climatic definition, in which Summer is the hottest three months of the year, Winter is the coldest three months, and Spring and Autumn are the moderate months in between them. The upshot of this is that each season starts a month after it does under the sunlight definition.

The astronomical definition of the seasons holds that the seasons begin *on* the solstices and equinoxes. This sounds like it would primarily be of interest to astronomers, but for some reason it's also the formal legal definition of the seasons used in America and France. It's also used in Iran and Afghanistan, but they actually have a reason for it since the northward equinox is also the day of #Nowruz.

Equatorial cultures and civilisations also tend not to bother with more than two seasons. Many traditional African cultures as well as the #Quechua (AKA #Inca) just have a dry season and a rainy season, because sunlight and temperature don't change much across the year close to the equator, and cycles of rainfall are more important and noticeable.

It is known that pre-Avestan Iranian cultures also just had summer and winter, and the same may have been true of the earliest Sumerians. Under this system, starting the year with one of the equinoxes makes sense, since it marks the beginning of the brighter (Spring) or darker (Autumn) half of the year.

However, this is not universal; #IntiRaymi, the #Quechua New Year celebration, was traditionally tied to the Winter Solstice.

Ancient #Egypt famously had three seasons in the year, all of which were based on the flooding of the #Nile. There was the Flood season, the Planting season, and the Reaping season. These were kind of sort of tied to the heliacal rising of #Sirius.

In India, to this day, there are *six* seasons - Vasant (Spring), Grishma (Summer), Varsha (Monsoon), Sharad (Autumn), Hemant (no direct translation), and Shishir (Winter).

I hope this illustrates that, while seasons are ultimately based on astronomy, they are still human constructs.

Personally, I intellectually prefer the Chinese system because it's rigorous, but I do have an emotional and cultural attachment to the Celtic system.