Figure: The Earth's seasons understood astronomically

Caption: The image shows the Earth's orbit around the Sun and how the Earth's season are determined astronomically.

Further explication:

1. The image is not-to-scale.

2. Over relatively short time periods like a year and a human lifetime, the direction of the Earth's axial tilt is constant with respect to the observable universe, and therefore also with respect to ecliptic pole which is perpendicular to the ecliptic plane which is the plane of the Earth's orbit.

   Note, the observable universe defines the fundamentally absolutely unrotating reference frame with respect to which all inertial frames do NOT rotate---except maybe in very strong gravitational fields like near black holes???---but yours truly has to guess about this since no one explicates this factoid. The best explication so far (and it does NOT say much) is Wikipedia: Inertial frame of reference: General relativity.

3. The varying average amounts of solar intensity (AKA insolation AKA solar energy per unit time per unit area) received by the Northern Hemisphere Southern Hemisphere are the causes of the seasons.

4. The Northern Hemisphere is in summer when the north end of the Earth's axis is tilted toward the Sun and in winter when it is tilted way.

5. In between summer and winter, one has the in-between seasons, spring and fall.

6. The Southern Hemisphere is the mirror-image case to the Northern Hemisphere.