Earth’s orbit around the Sun is an ellipse and an ellipse has apsides. The point when it is closer to the Sun (called Perihelion) happens in plain Northern Hemisphere’s winter (roughly one of the first days of January) whilst the point of its further distance (called Aphelion) falls in plain Northern Hemisphere’s summer (roughly one of the first days of July).
It has been observed that the average temperature of the planet is around 4ºC higher in Aphelion, though the intensity of sunlight is less, due to increased distance.
The mystery revealed
The reason for that is in the physics of water. In Perihelion, the Southern Hemisphere is being warmed up by an increased summer solar radiation. But the Southern half of the planet is mostly made of water – as opposed to the Northern Hemisphere which has more land. Water will take longer than land to heat up. And, while this is happening, the Earth is colder. In Aphelion, the larger mass of Southern water has been heated for a whole summer and is releasing the heat as slowly as it has accumulated it. Even though, during the Northern Hemisphere’s summer, Earth receives less solar radiation, it’s being kept warm by the heat reserve in the Southern waters which act as a radiator.
This is exactly the same reason why the sea feels warmer at the sunset: it’s not receiving solar heat anymore but it has accumulated it during the day, and the beach is getting colder quicker….
Would you have thought that what everybody notices when on the beach actually happens at a planetary level?