The Tilt of the Earth and the Seasons

The Earth travels around the Sun once in about 365¼ days — one year. As it travels, its axis stays tilted at about $23.5^\circ$ and keeps pointing in the same direction in space all the way round.

That unchanging tilt is what produces the seasons. At one point in the orbit the southern hemisphere leans towards the Sun; six months later it leans away.

When your hemisphere leans towards the Sun, two things happen together:

1. the Sun climbs higher in your sky, so its energy strikes the ground more directly and is concentrated on a smaller area;
2. your daylight period is longer, so the ground is heated for more hours.

Both effects push in the same direction — and you get summer.

In South Africa: around 21 December the southern hemisphere leans towards the Sun. That is our longest day and the middle of our summer — Highveld thunderstorms, December holidays. Around 21 June we lean away: our shortest day and the depth of winter. Around 21 March and 23 September neither hemisphere leans towards the Sun; these are the equinoxes, when day and night are close to equal everywhere.

Worked example — day length in Cape Town. The longest day gives about 14 hours 25 minutes of daylight; the shortest about 9 hours 55 minutes. In minutes:

The longest day has about 270 minutes — four and a half hours — more daylight than the shortest.