Why isn't the first day of fall...

on the same date every year? Sometimes it’s on September 22, but other years it’s on the 21st or 23rd. What gives?

Because it depends on the day that the autumnal equinox actually occurs. It’s not like Congress legislates these things.

It is. 1st March every year.

Unless you consider Labor Day to be the first day of fall, in which case Congress does legislate these things.

But to the question the OP was probably asking, it’s not the equinox shifting relative to our calendar, but our calendar shifting relative to the equinox. Leap years are the biggest reason for this.

Why would the first day of autumn be on a certain date? Seasons are determined by temperature, not calendars.

We have a leapyear roughly every four years, when a full day is added to the end of February to account for the accumulation of about a day of error, over the previous four years, between our calendar and the phase of earth’s orbit around the sun. Correspondingly, the important points of the earth’s orbit - spring/fall equinox, and winter/summer solstice - also shift around on the calendar.

Note that those solstice andequinox events are points - instantaneous moments in time - rather than full days. So even though there’s only a quarter of a day change in the timing of any of these events on the calendar, you may see it switch backward one day on the calendar if there’s a leap year, and then one day forward the next year if it’s being dithered back and forth across a midnight.

A season is 1/4 of the trip around the sun. Temperatures are irrelevant.

Astronomical fall begins on the equinox, which is defined as the moment when the center of the sun is in the same plane defined by the Earth’s equator.

However, there is also meteorological fall, which is defined by dates. It starts September 1.

If seasons were defined by temperature then we would have had a few days of fall last week, and we’ll have a couple of days of summer next week, and we might even get a couple of days of spring in January.

[li]Meteorological summer starts when the mean temperature over 24 hours has been above 10 C during a period of 5 days.[/li][li]Meteorological autumn starts when the mean temperature over 24 hours has been below 10 C during a period of 5 days.[/li][li]Meteorological winter starts when the mean temperature over 24 hours has been below 0 C during a period of 5 days.[/li][li]Meteorological spring starts when the mean temperature over 24 hours has been above 0 C during a period of 5 days.[/li][/ul]
Dates are irrelevant.

A season is experiential. It has nothing to do with astronomy or meteorology. Fall has begun when it feels like it is fall.

There are many definitions of the seasons. Meteorological seasons are location specific, not used by many people, and not the subject of the OP.

So do they have Spring, Summer, Fall, and Winter in Cameroon?

We have much to learn from them.

According to a paper available from the American Meteorological Society, written by Kevin Trenberth of the Department of Atmospheric Sciences at University of Illinois at Urbana-Champaign, meteorological seasons are based on dates.

Cecil (and/or his climate-researcher source) appears to agree with you:

Cecil also weighs in against the idea that the first day of summer is “officially” on the solstice (and, by implication, that the first day of autumn must be the equinox).

So does fall start earlier in Toronto than it does in Atlanta?

If we somewhat arbitrarily declare the seasons be of equal length, then a reasonable start date for autumn would be [equinox, minus half of three months, plus temperature lag due to oceans retaining heat]. The latter varies by location but is typically about one month. That gives a date of about September 8th. But September 1st seems close enough to me.

Why not?

In Bengal, there are six seasons – summer, monsoon, autumn, dry season, winter, spring

Yes, but you just can’t tell them apart as easily as in some other areas, because the weather isn’t the leading indicator of the season in that given area.


Yes, yes, a thousand times yes.