Why Are Summers Hotter in the Southern Hemisphere?

 It's actually pretty simple. Earth's orbit is not a circle around the sun.

Yesterday was perihelion day. This is the day on which the Earth is closest to the sun, and it's always in the first week of January.

Aphelion day, its opposite, is somewhere in late June and early July.

Summers in Australia are hotter because for them, perihelion coincides with their part of the planet being tilted towards the sun.

It's a small effect, though; the Earth's axial tilt has far more of an impact on weather than the orbit.