Earth's axis is tilted 23.5 degrees, and it keeps pointing the same direction in space all year.
Step 1: Something to Notice
Watch first. The explanation comes later.
In July it is summer in Canada and the middle of winter in Chile. Both places are on the same planet, the same distance from the Sun, on the very same day.
How can two places on one planet have opposite seasons at the same moment?
Step 2: Find Out
Run a full year and watch two numbers: the distance, and the sunlight energy for each half.
Step 1 — Predict
Across one year, which number do you think changes more — the distance to the Sun, or the sunlight energy hitting the northern half?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: Find Out
Set the tilt to zero and see whether there are any seasons left.
Step 1 — Predict
If Earth had no tilt at all, what would happen to the seasons?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 4: So Here Is Why
Now the explanation, after you have seen it happen.
Leaning toward or away
For half the year the northern hemisphere leans toward the Sun. For the other half it leans away.
The angle is what matters
When sunlight arrives steeply it is concentrated on a small patch of ground. When it arrives at a shallow angle the same light is spread over a much bigger patch, so each part gets less.
Distance barely matters
Earth's orbit is very nearly a circle. The distance changes by only about 3 percent, and Earth is actually closest to the Sun in January.
Opposite halves
Because the two hemispheres always lean opposite ways, their seasons are always opposite.
It is the tilt, not the distance
The Earth is actually closest to the Sun in January. Seasons come from the 23.5 degree axial tilt, which changes how directly sunlight strikes each hemisphere and how long the day lasts.
The hemispheres are opposite
When the northern hemisphere leans towards the Sun it is summer there and winter in the south. If distance caused seasons, both hemispheres would have summer at the same time — and they do not.
Step 5: A Common Mistake
Lots of people think
“It is summer when Earth is closer to the Sun and winter when it is further away.”
Step 6: Why High Latitudes Have Extreme Seasons
The same idea somewhere new.
Near the equator the sunlight angle barely changes across the year, so there is little seasonal difference. Near the poles the change is enormous — which is why northern Norway has months of daylight in summer and months of darkness in winter.
Step 7: Think It Through
Practice makes it stick.
The Two Cities
Problem 1 of 2
It is June. Sydney is in the southern hemisphere. Is Sydney having summer or winter? Enter 1 for summer, 2 for winter.
The Distance Check
Problem 2 of 2
Earth is 152 million km from the Sun in July and 147 million km in January. What is the difference, in million km?
Tilt, Angle, and Energy
1 of 6
What is Earth's axial tilt, in degrees?
2 of 6
If the tilt were 0 degrees, how many seasons would there be? Enter 0 for none.
3 of 6
In which month is Earth closest to the Sun?
4 of 6
A hemisphere is leaning away from the Sun. Sort what follows.
Tap something to move it.
- Empty
- Empty
5 of 6
Record the sunlight energy for each hemisphere from the model.
| Northern half | Southern half | |
|---|---|---|
| January | 37 | |
| July | 37 |
All values in percent.
6 of 6
By roughly what percentage does the Earth-Sun distance change across a year?
Step 8: Quick Check
Show what you know.
Question 1 of 1
What causes the seasons?
Step 9: Explain It in Writing
Claim, evidence, then reasoning.
The question
It is January. Earth is at its closest point to the Sun for the whole year, and it is deep winter across Canada. Explain what causes the seasons, using evidence from the model.
Words to useaxial tiltanglehemisphereconcentrateddistance
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- Earth's axis is tilted 23.5 degrees and keeps pointing the same way all year.
- Seasons come from the angle sunlight strikes at, not from distance.
- Earth is actually closest to the Sun in January, during northern winter.
- The two hemispheres always have opposite seasons.