Why is summer warm?

On 21 June, at about 41° north, the Sun is up for 14 h 57 min, and at noon it climbs 72° above the horizon. Change the date or the place, here or on the drawings, and this whole page rewrites itself for that day.

Ask why summer is warm and a common answer is that the Earth must be closer to the Sun. Before you read on, make a guess: in which month is the Earth nearest the Sun? Tap one here, or a month on the orbit.

Earth is nearest the Sun around 3 January, in the depth of the northern winter, and farthest around 4 July. The gap is about five million kilometres, three per cent of the distance, and it changes the sunlight reaching us by about seven per cent.³ That is real, but it cannot be what makes the seasons: it is the same for the whole planet at once, yet the north and the south have opposite seasons, and in the north the nearest point even falls in winter. Something else is going on.

A lean that never changes

The something is a lean. Earth spins about an axis that is tilted 23.4°,² and the axis keeps pointing the same way in space all year round, toward the Pole Star.⁵ So on one side of the orbit the north leans toward the Sun, and six months later it leans away.

On that date the north leans toward the Sun. The Sun lights the left half of the globe, and the line between day and night runs straight down its middle. Your latitude is the red circle, and the part of it in sunlight is your day: 14 h 57 min of the twenty-four. In June the same circle swings into the light and the day stretches to 14 h 57 min; in December it shrinks to 9 h 03 min.

Near the poles this becomes extreme. Inside the Arctic Circle there are days in June when the circle never leaves the light, and the Sun does not set at all. At the equator the circle is always cut in half, and every day lasts twelve hours.

The Sun’s path across your sky

On that day, by sun time, the Sun rises at 4:31 in the east-northeast, climbs 72° above the horizon at noon, and sets at 19:29 in the west-northwest. Drag the Sun across the sky, or ▶ play the day. The dashed arcs are its paths at midsummer and midwinter.

Watch your shadow. At noon on that day it is 0.3 times your height; at midsummer noon it shrinks to 0.3 times, and at midwinter noon it stretches to 2.1 times. A higher Sun and a longer path across the sky are the two halves of summer.

The same light, spread thinner

Longer days are half the story. The other half is the angle. At noon on that day the Sun stands 72° above your horizon, so a beam of sunlight one metre wide lands on 1.05 m of ground, and each square metre gets 95% of what a Sun straight overhead would give it.

In June the noon Sun climbs to 72° and each square metre gets 95%; in December it reaches only 26°, and the same beam is spread so thin that each square metre gets 43%. More light per square metre, for more hours a day: that is summer.

What if the Earth did not lean?

Earth’s axis tilts 23.4°. Drag that number down to zero and watch the globe: the red circle is cut exactly in half on every date, every place away from the poles gets twelve hours of daylight every day of the year, and the seasons as we know them disappear. All that is left is the small swing of about seven per cent as Earth’s distance from the Sun changes,³ the same for the whole planet at once. Drag it the other way, toward 98°, and you have Uranus, which spins on its side.⁶

Your year

Here is the whole year at 41° north: the hours of daylight month by month, and below them the distance to the Sun, which rises and falls out of step with the seasons.

Try it yourself

Everything you need is on this page. Each task ticks itself off when the page above is set up to show it.

1. Make the Sun stay up all day and all night.Above the Arctic Circle, around midsummer, your whole circle of latitude stays in the light: the Sun circles the sky without setting.
2. Find a place and a date where, at noon, you cast no shadow at all.Between the tropics the noon Sun passes straight overhead twice a year. On the Tropic of Cancer it happens once, on midsummer’s day.
3. Get rid of the seasons without moving the Earth one kilometre.With no tilt, every place away from the poles gets twelve hours of daylight and the same noon Sun all year. Distance alone leaves only a small swing, the same everywhere at once.
4. Find a winter noon when each square metre gets less than a third of full sunlight.When the noon Sun sits below about 19°, the same beam is stretched over more than three square metres of ground.
5. In your own words: why is it colder in winter?

So summer is warm because, for a few months, your part of the world leans toward the Sun: the Sun climbs higher, its light falls more squarely on each square metre, and it stays up longer.