Is it just coincidence that the 4 season begin and end at same point in the orbit around the sun?
In: Physics
Your question have an incorrect assumption and that is that the year is based on the orbit but is is not. It is based on the axial direction of the earth’s relative sun. They are close but not the same. There is a difference of 20 minutes per year. The obit definition is often used because it is simpler and works well enough in most cases but not in your question.
The first question is what is a year? There are many years:
A sidereal year is when the position of the sun is identical relative to the stars in the sky. This is one orbit around the sun but it is not what our calendar is base on.
A tropical year is based on the direction of the axis of the earth relative to the sun. So on the equinox, the equator is parallel to the orbital plane of the sun. This is what our calendar is based on because it will correspond with the seasons. It is 20 minutes shorter than a sidereal year.
So the calendar matches the season because both is a result of the direction of the earth axis relative to the sun. The calendar is not directly based on an orbit around the sun.
The axis of the Earth is very quite stable. There is [axial_precession](https://en.wikipedia.org/wiki/Axial_precession) like a gyroscope but the period is 25,772 years. It is this change that results in the 20 minutes
So between two years, the position of the stars relative to the sun is quite close on the same day but not identical.
So there is a slow change and the position of the start for the same time of year changes slowly. They will move 1 degree every 71.6 years. So on the equinox, the stars are 1 degree of from where they were in 1949
So in most cases, it is enough just to explain a year with the orbit around the sun but for very exact answer you have to include that axial precession and the difference between a sidereal year and the tropical year.
If you look at the [dates of the Zodiac](https://en.wikipedia.org/wiki/Zodiac#Table_of_dates) you can see that the used dates and the real position of the stars do not match up. The common Ptolemaic tropical zodiac is based on the location of consolation around 2000 years ago and does not match what you observe today.
This is because of the difference between a sidereal year and the tropical year.
A day that in normal use is a solar day is not one rotation of the earth around its axis but the time the sun is in the same sport in the sky. We move around the sun. Even if the earth did not rotate the sun would have move one revolution in the sky. So a day is the combined effect of earth axial rotation and orbit around the sun.
So a sidereal day the is stars return to the same position ie one revolution of earth is 23 hours, 56 minutes, 4.0905 seconds the extra time ~4 minutes is from the orbit around the sun.
One revolution of the sun per year because of the obit around the sun is approximately 24*60/365= 3.94 minutes per day. It and the sidereal day added to result in a solar day,
So a solar day is 24 hours and earth rotates a bit more than one revolution per day.
So it is not a coincidence because the calendar is not based on orbit by axial changes of the earth. The same season will start on the opposite side of the sun after 12886
So like most concepts in science you can get a bit simplified explanation that works most of the time like 1 year = 1 orbit around the sun. It is often good enough, but for more advanced questions like the one you asked you have to look closer with a more exact but complex explanation.
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