Why are there seasons?

Subjects

physics, geography, astronomy

Age range

10 to 14 years old

Difficulty

medium

Lesson time

45 minutes

Material

• A computer or tablet per group, with a recent web browser (the scene uses WebGL).
• The printed pupil sheet below.

Scene to open

Earth’s seasons

• Explain that the seasons come from the tilt of the Earth’s axis, not from its distance to the Sun.
• Link the latitude where the Sun stands overhead to the equinoxes and solstices.
• Read information from a dated 3D scene and use it to test a prediction.

1. An idea to test

Ask the class why it is hotter in summer. Many pupils answer that the Earth is then closer to the Sun: this is the idea the activity tests. Have them write their prediction down before the tour. ESA’s sheet “One year on Earth” (PR45) builds the same reasoning with a torch and a polystyrene ball; Galaxy shows it on the real Earth, at the real dates.

Answer key

In the northern hemisphere, is the Earth closer to the Sun in summer or in winter?

In winter. The Earth passes closest to the Sun in early January, between January 2 and January 5, and farthest in early July, between July 3 and July 6. The difference is only about 3% of the distance, and the closest point falls in the middle of the northern winter.

To go further, advanced: The Earth is about 3% closer to the Sun in January than in July. By how much does the sunlight it receives increase, and why does that not make summer?

The flux varies as the inverse square of the distance: about 7% more at perihelion. The gain is real, but the northern hemisphere receives it in the middle of its winter: the height of the Sun and the length of the day, set by the tilt, outweigh it (in Paris, 16 h 11 min of daylight in June against 8 h 15 min in December).

2. The Earth side-on

Play the tour “Earth’s seasons” with the link below, on a projector or on each group’s screen. Galaxy shows the Earth side-on, the Sun to the right, at the four real turning points of the coming year, then at perihelion. Point out that the axis keeps the same direction while the Earth goes round the Sun, like the stick through the ball in ESA’s model.

Answer key

By how much is the Earth’s axis tilted from the perpendicular to the plane of its orbit?

By 23.44° according to the cited source; the Earth’s card shows it rounded to one decimal place, 23.4°.

At the June solstice, is the north pole in daylight or in darkness? And at the December solstice?

At the June solstice it stays in daylight for a whole turn of the Earth: everything north of the Arctic Circle, at 66.56° N, sees the Sun around the clock. At the December solstice it is the polar night.

Above which latitude is the Sun overhead at noon, at the equinoxes and at the solstices?

At the equinoxes, above the equator (0.0° in March, 0.0° in September). At the June solstice, above the Tropic of Cancer, at 23.4° N. At the December solstice, above the Tropic of Capricorn, at 23.4° S. These latitudes equal the tilt of the axis, 23.44°: the tilt makes the seasons.

To go further, upper secondary: Why is the Arctic Circle at 66.56° N? Relate it to the tilt of the axis.

The Sun lights every point less than 90° away from the point where it stands overhead. At the June solstice that point is at 23.4° N, so the lit half reaches 90° beyond it, over the pole: everything north of 90° minus the tilt, 90° − 23.44° = 66.56°, stays in daylight for a whole turn.

3. Discussion

Have the groups answer using what they saw. These two questions are those of ESA’s sheet; the answer key gives the figures Galaxy computes for Paris.

Answer key

Why is it warmer in Paris in summer than in winter? Give two reasons.

The Sun climbs higher in the sky, so its rays arrive more steeply and heat each square metre more; it is overhead at 23.4° N at the June solstice. And the days are longer: in Paris, 16 h 11 min of daylight at the June solstice against 8 h 15 min at the December solstice.

It is winter in France and you want a warm beach. Where would you go, and why?

To the southern hemisphere: at the December solstice the Sun is overhead at 23.4° S, and it is southern summer.

Figures computed by Galaxy for the years 2026 to 2035, with the same code as the scene; dates in Universal Time. Quoted facts:

• Earth, axial tilt: NASA NSSDCA, Planetary and Satellite Fact Sheets, read on September 17, 2026.

Why are there seasons?

Address of the scene: https://galaxy.adrianguichard.dev/earth/?tour=seasons

1. An idea to test

1. Before the tour, write down your prediction.

In the northern hemisphere, is the Earth closer to the Sun in summer or in winter?

To go further, advanced: The Earth is about 3% closer to the Sun in January than in July. By how much does the sunlight it receives increase, and why does that not make summer?

2. The Earth side-on

1. Open the tour with the link on this sheet.
2. At each step, look at the north polar cap: is it on the day side (right) or on the night side (left)?
3. Open the Earth’s card (the panel that appears on the right) and find its axial tilt.

By how much is the Earth’s axis tilted from the perpendicular to the plane of its orbit?

At the June solstice, is the north pole in daylight or in darkness? And at the December solstice?

Above which latitude is the Sun overhead at noon, at the equinoxes and at the solstices?

To go further, upper secondary: Why is the Arctic Circle at 66.56° N? Relate it to the tilt of the axis.

3. Discussion

1. Discuss in your group, then answer.

Why is it warmer in Paris in summer than in winter? Give two reasons.

It is winter in France and you want a warm beach. Where would you go, and why?

The layout follows ESA’s “teach with space” sheets, which the ESERO network relays to teachers. Reference read on October 18, 2026: ESA Education, One year on Earth: Understanding seasons (teach with space, PR45).

All activities