Seasons

In summer you swim in the outdoor pool, in winter you might build a snowman. But why does that change at all? Many people think the Earth is closer to the sun in summer. That sounds logical, but it is wrong. In fact, the Earth is even slightly closer to the sun in January than in July. The real reason for the seasons is something far more elegant: a tilted axis.

The tilted axis of the Earth

The Earth spins like a top around its own axis. But this axis is not upright: it is tilted by about 23.5 degrees, and it points in the same direction in space all year round. As the Earth travels around the sun once a year, first one half of the Earth and then the other is tilted towards the sun. When your hemisphere, the half of the Earth you live on, is tilted towards the sun, you have summer: the sun climbs high at noon, its rays hit the ground much more steeply and heat it up strongly, and the days are long. Half a year later your hemisphere is tilted away from the sun: the sun stays low, the same energy is spread over a larger area, and the days are short. That is winter. In between lie spring and autumn: at the start of each, neither hemisphere is tilted towards the sun, and day and night are almost equally long everywhere.

Two hemispheres, swapped seasons

When the northern hemisphere is tilted towards the sun, the southern hemisphere automatically points away from it. That is why the seasons on the two hemispheres are always exactly swapped: while you celebrate Christmas in the snow in December, people in Australia are lying on the beach, because it is the height of summer there. And when the summer holidays start here in July, it is winter in Argentina and South Africa. Only at the equator, exactly between the hemispheres, does the midday sun stay high all year round, so it stays similarly warm there the whole time.