What happenes when a season changes?

Answer 1

It is the tilt of the Earth's axis at 66.5 degrees to the ecliptic in combination with the rotation of the planet around the Sun, that are the essential parts of seasonal changes.

The first thing that needs clarification is that the tilt relative to the true North Pole and the plane of the Ecliptic, is always pointing at the same Star in outer space, no matter what the season. The ecliptic is the enclosed area traced by the Earth as it moves around the Sun and as it is ~circular, it can be divided into four quarters, each of 90 degrees. (Not to be confused with Lunar quarters which are monthly where-as these are annual.)

The easiest way to visualize this, is to put yourself on the North Star at which the Earth's North Pole points, and look back at the Ecliptic - the Earth - and the Sun. The Earth's North Pole will be pointing straight at you all the time. Unfortunately you have to choose an arbitrary "up" and I like to see the ecliptic with the top as the March equinox. That means half way between mid-summer and mid-winter for the Southern Hemisphere, and moving towards Winter.

(When looking from the North Star, the Earth is moving counter-clockwise around the Sun and this counter-clockwise direction would be reversed if we were looking outwards from the inner galaxy onto the South Pole.)

Obviously the bottom of the ecliptic is the September equinox. Half-way between mid-winter and mid-summer for the Southern Hemisphere and moving towards Summer.

90 degrees of this rotation is also called three months and three months after each of these equinoxes, each of the hemispheres will be in either Mid-Winter or Mid-Summer. So the equinoxes were either Autumnal or Spring depending in which hemisphere you live.

It is also said that the equinox is the moment at which the Sun crosses the Equator whether going North or South. This may or may not be helpful as that moment may be night or day, so saying that it occurs on a particular day is a minor misrepresentation. Not to mention it's timing relative to the International Date Line. In reality the Equator of the Earth is crossing the Ecliptic and given the 66.5 degree tilt of the Earth's axis relative to that Ecliptic, the duration of transit would very short indeed.

I offer no apologizes for supplying a description that may be beyond the comprehension of some, but it is obviously not a simple question.