Why is summer warm?

There is a second clue that the distance story can’t be right. When it is summer in London or New York, it is winter in Buenos Aires and Sydney. Both hemispheres are on the same planet, at the same distance from the Sun, yet they have opposite seasons. Whatever causes summer must treat the north and the south differently.

This page builds the real answer one piece at a time. First, let’s look at how much Earth’s distance from the Sun actually changes.

The orbit, drawn to scale

Earth goes round the Sun on an ellipse, a slightly squashed circle, with the Sun not at the centre but a little to one side (at a point called a focus). The closest point of the orbit is called perihelion; in 2026 it came on 3 January, at 147.1 million km. The farthest, aphelion, came on 6 July, at 152.1 million km.

Five million kilometres sounds like a lot, but it is only about 3% of the distance. In the figure the orbit is drawn to scale beside a perfect circle (dashed). Drag Earth round its orbit, or press Play, and watch the distance and the strength of the sunlight arriving at the top of the atmosphere. Can you see which is the ellipse?

The orbit is so nearly round that you cannot tell it from the circle by eye. The Sun sits just 2.5 million km off-centre — 1.7% of the orbit’s radius, a shift of two or three pixels here.

Sunlight does get stronger when we are closer. Light from the Sun spreads out over the surface of an ever-bigger sphere, so its strength falls with the square of the distance. At the average distance, each square metre facing the Sun above the atmosphere receives about 1,361 watts — a number called the solar constant. (A watt is a joule of energy per second; one square metre facing the Sun gets roughly the power of a small electric kettle.) In January that rises to about 1,408 W/m²; in July it falls to about 1,317 W/m². That is a real difference of about 7% — and it runs the wrong way for the northern summer.

Now look at the little arrow on Earth. It is Earth’s axis — the line through the North and South Poles — seen from above. The axis is tilted by 23.44° from upright, and as Earth goes round the Sun, it keeps pointing the same way in space (towards the star Polaris). In June the north end leans towards the Sun; in December, six months round the orbit, the same fixed tilt means the north end leans away. That is easier to see from the side.

The figure below shows the orbit from a little above, with June on the left and December on the right (the four pale Earths mark the solstices and equinoxes). Press Play, or drag Earth round, and watch the axis. It never turns: it leans the same way — to the right in this picture — all year. What changes is where the Sun is. Notice which hemisphere is turned towards the Sun in June, and which in December.

In June the northern hemisphere is tipped towards the Sun and the Sun passes overhead at noon as far north as 23.44° N (the Tropic of Cancer). In December the southern hemisphere gets its turn, and the Sun is overhead at 23.44° S (the Tropic of Capricorn). At the equinoxes in March and September, the axis leans sideways relative to the Sun, neither towards nor away, and the Sun is overhead at the equator.

That tilt is the whole story. The next two figures show the two ways it matters.

Steep sunlight is strong sunlight

Shine a torch straight down at a table, and you get a small, bright circle. Tilt the torch and the same light spreads into a long, dim oval. Sunlight works the same way. The same beam of sunlight covers more ground when the Sun is low in the sky, so each square metre gets less of it.

The figure shows a beam of sunlight 1 metre wide. Drag the Sun, or use the slider, to change its elevation — its angle above the horizon. Watch how long the lit patch of ground becomes, and what happens to the power landing on each square metre.

At noon in London (latitude 51.5° north) the Sun stands about 62° high in late June, but only about 15° high in late December. The same beam then covers a patch 3.4 times as long, so each square metre of ground gets less than a third as much power. The rule is simple:

power per m² of ground = power per m² of beam × sin(elevation)

The sine of an angle here is just the ratio “beam width ÷ patch length”: 1 when the Sun is overhead, about 0.26 at 15°, and 0 at the horizon. Compare that factor of 3.4 with the 7% from the changing distance.

Long days and short days

The tilt has a second effect, just as big. Earth spins once a day, and at any moment half of it is lit. The line between day and night is called the terminator. Because the axis is tilted, the terminator does not run from pole to pole. In June it slices the globe so that most of each northern circle of latitude lies on the day side: a place there spends more than half of each turn in sunlight.

In the figure, the Sun is to the left. Pick a latitude and a date, and the highlighted circle shows the path your place travels during one day. The thick brass part is daylight; the rest, on the shaded side, is night. Try the Arctic Circle (66.6° north) in June and in December, and try the equator.

At London’s latitude, the longest day has about 16 h 24 min of sunlight and the shortest about 7 h 36 min. North of the Arctic Circle the Sun does not set at all around the June solstice (the midnight sun, or polar day), and does not rise around the December solstice (polar night). At the equator, every day is close to 12 hours long all year. And look at the southern latitudes: everything is flipped, which is why the seasons are opposite in the two hemispheres.

The two effects work together. In summer the Sun climbs higher, so each hour of sunshine delivers more power to each square metre, and it shines for more hours.

Putting it together

We can now add everything up. For any latitude and day, the model works out where the Sun is overhead (from the tilt and the date), how long the Sun is up, how high it climbs through the day, and how far away the Sun is. From that it computes the daily insolation: the sunlight arriving at the top of the atmosphere on one square metre of level ground, averaged over 24 hours. (Insolation is short for “incoming solar radiation”.)

The solid line is the real Earth. The two switches let you remove one cause at a time. “No tilt” stands Earth’s axis upright, leaving only the changing distance. “Circular orbit” removes the changing distance, leaving only the tilt. Compare how much each one moves the line. Then try the tilt slider, and the southern hemisphere.

At 40° north — the latitude of Madrid, Beijing and Philadelphia — the daily sunlight rises from about 156 W/m² in late December to about 483 W/m² in late June: three times as much. With the tilt removed, the swing shrinks to about 22 W/m², and it peaks in January. With the tilt kept but the orbit made circular, the curve barely moves: the June peak rises from 483 to 499 W/m², and the December low drops from 156 to 151. The tilt is the cause; the distance is a small correction.

That correction is not zero, though. It adds to the tilt’s effect in the south, where summer falls near perihelion, and subtracts in the north. Switch to 40° south and you will see a slightly higher peak — about 516 W/m² in late December — and a slightly lower trough. (In the real world, the southern hemisphere is mostly ocean, which smooths its seasons out anyway.)

Two more things to try. Set the latitude to 90°: in late June the North Pole receives more sunlight per day than the equator ever does (about 524 W/m² against at most about 440), because its Sun never sets. And drag the tilt to zero: every day of the year becomes the same, apart from the gentle wobble of the distance. With no tilt, there would be no seasons worth the name.

So, why is summer warm?

Because Earth’s axis is tilted. For half the year your hemisphere leans towards the Sun, so the Sun climbs higher in the sky, its light falls more steeply and less spread out, and it stays up for more hours each day. Six months later your hemisphere leans away, and the opposite happens. Our distance from the Sun does change by about 3%, and our sunlight by about 7%, but in the north that change runs against the seasons, and it is far too small to cause them.