Seasons

It is natural to think summer comes when the Earth is nearer the Sun. It does not. The Earth is nearest the Sun in early January, in the depth of the British winter.

The real cause is that the Earth leans. Its axis is tipped over by 23.4°, and it keeps pointing the same way in space all year, towards the same stars.

So in June the northern half of the world leans towards the Sun. The Sun climbs high, the days are long, and the ground has time to warm. Six months later the Earth is on the other side of its orbit, still leaning the same way, which now means the north leans away. The Sun stays low and the days are short.

The southern half of the world gets the opposite. Christmas in Australia is in midsummer.

The tilt does two things at once, and they add together.

It changes how high the Sun gets. Take Manchester, at latitude 53.5° north. At noon on the June solstice the Sun stands 60° above the horizon. At noon on the December solstice it reaches only 13°. Low sunlight is spread thin: a beam arriving at 13° covers nearly four times as much ground as the same beam arriving at 60°, so each square metre gets about a quarter of the warmth.

It changes how long the Sun is up. In Manchester the Sun is above the horizon for about 17 hours in late June and about 7½ hours in late December.

Distance hardly comes into it. The Earth’s orbit is so close to a circle that its distance from the Sun changes by only 3% through the year, from 147.1 million km in early January to 152.1 million km in early July. Sunlight is about 7% stronger in January than in July, which takes a little of the edge off northern winters and adds a little to southern summers.

The tilt also draws four lines on the globe. The tropics, at 23.4° north and south, are the furthest places where the Sun can stand straight overhead. The polar circles, at 66.6°, are the edges of the regions that get at least one day of midnight Sun and one day with no sunrise.

The hottest and coldest weeks come a month or so after the solstices, because the ground and above all the sea take time to warm up and cool down.

Other planets show what tilt alone can do. Mars leans by 25.2°, much like the Earth, and has four seasons that each last about twice as long as ours. Venus is almost upright and has no seasons to speak of. Uranus lies on its side at 97.8°, so over its 84-year orbit each pole gets about 42 years of sunlight followed by 42 years of dark.