Why the seasons are reversed in the southern hemisphere ?
Simply because of the tilt of the Earth.
Because of this inclination, the northern hemisphere, receives more energy from the Sun, While the southern hemisphere receives less.
This corresponds to the boreal summer (in the northern hemisphere) and the southern winter (in the southern hemisphere).
Six months later, It's the opposite because the southern hemisphere is better exposed to solar radiation.
In spring and autumn, the radiation received in the two hemispheres is identical. But the temperature is different, because a hemisphere coming out of winter and its soil and water have yet to be reheated, While the other hemisphere comes out of the summer and still enjoys its soil and water heat.
What's better than this video of “It's not rocket science” to explain it in detail …
And voila !