Why do seasons occur?
Seasons primarily result from Earth's tilted rotation axis. Solar angle and daylight duration change through the year. Changes in Earth-Sun distance are not the main explanation for opposite seasons occurring simultaneously in the two hemispheres.
Solar angle and daylight length
When a hemisphere tilts toward the Sun, sunlight strikes more directly and days last longer. Energy is concentrated over less area and received for longer, generally strengthening heating. The other hemisphere experiences the opposite. Equinoxes and solstices mark particular geometries. Earth's axis maintains approximately the same direction in space during a year while its orientation relative to the Sun changes.
Are seasons identical everywhere?
Ocean heat storage helps delay peak warmth until after the summer solstice in many places. Latitude, elevation, maritime influence, and circulation shape local seasons. Wet and dry seasons can be more prominent than temperature changes in the tropics. Astronomical and meteorological season definitions differ. Axial tilt explains the annual cycle but not each day's weather or every regional rainfall pattern.