It is January in Israel. What season is it in Argentina?

Argentina is in the southern hemisphere. The seasons in the two halves are opposite. Summer

Why does a low-angle ray heat less?

The same energy reaches the ground. At a low angle it is smeared over a larger area. Each square metre receives less. Because the energy is divided over a bigger area

Israel has about 14 hours of daylight in June and 10 in December. What is the difference, and what explains it?

14 minus 10 is 4. In summer our half leans towards the Sun. The same tilt sets both the ray angle and the day length. About four hours, and one single tilt explains both

Earth and Space · Grade 6

Is summer hot because the Earth is closer to the Sun?

No. The simplest evidence is that Australia has summer exactly when Israel has winter, and both are on the same ball at the same distance from the Sun. What decides it is the tilt of the axis: in summer the Sun's rays strike the ground steeply, concentrating more energy into a smaller area, and the daylight hours are longer.

Learning objectives

Explain why sunlight arriving at a steep angle heats the ground more than the same light arriving at a shallow angle
Refute the claim that summer comes when the Earth is closest to the Sun, using the seasons in the southern hemisphere
Say which hemisphere is in summer for a given position of the Earth along its orbit
Connect the number of daylight hours in Israel in June and in December to that same tilt

The refutation, in one line

If summer came from being closer to the Sun, then when it is hot in Israel it would have to be hot in Australia too — same ball, almost exactly the same distance.

What actually happens is the precise opposite. In July Israel has summer and Australia has winter; in January it reverses. Same ball, same distance, two opposite seasons.

That is enough to rule the explanation out. What is left has to be something that can differ between the two halves at the same moment — and that is the tilt.

The angle of the rays

Point a torch straight down at a table: you get a small bright circle. Tilt the torch: the same light is smeared over a larger area and goes dim.

That is exactly what happens to the ground. The same amount of energy concentrated into a small area heats it more; spread over a large area, each part receives less.

The Earth's axis is tilted by about 23.5 degrees and keeps pointing the same way all the way round its orbit. So for half the year the northern half leans towards the Sun and takes steep rays, and for the other half it leans away.

And the same tilt sets the length of the day

When our half leans towards the Sun, the line between light and dark crosses it so that we spend more hours in the light. In June Israel has about fourteen hours of daylight.

In December it all reverses and about ten are left. More daylight means more time to warm up, and fewer dark hours means less time to cool down.

The two effects push the same way: a steeper angle and more hours. That is why June is hotter than December, and not the distance from the Sun.

Worked examples

It is January in Israel. What season is it in Argentina?
1. Argentina is in the southern hemisphere
2. The seasons in the two halves are opposite
Answer: Summer
Why does a low-angle ray heat less?
1. The same energy reaches the ground
2. At a low angle it is smeared over a larger area
3. Each square metre receives less
Answer: Because the energy is divided over a bigger area
Israel has about 14 hours of daylight in June and 10 in December. What is the difference, and what explains it?
1. 14 minus 10 is 4
2. In summer our half leans towards the Sun
3. The same tilt sets both the ray angle and the day length
Answer: About four hours, and one single tilt explains both

Common mistakes

Explaining summer by closeness to the Sun: Then Australia and Israel would be hot together. They are on the same ball at the same distance and their seasons are opposite.
Thinking the axis changes direction during the year: It keeps pointing the same way. What changes is where the Earth is in its orbit, and therefore which side leans towards the Sun.
Separating the ray angle from the day length: Both come from the same tilt and both push the same way. In summer it is steeper and it is longer.

What to remember

Opposite seasons in the two hemispheres — distance is not the cause.
A steep angle concentrates energy, so it heats more.
The axis is tilted 23.5 degrees and keeps its direction.
The same tilt also gives longer daylight.