Lesson 2 of 4

The seasons.

About 30 minutes · 7 practice questions

Read the full article: Seasons

Why summer is hot and winter cold: not the distance from the Sun but the tilt of the axis. The noon Sun's height and the length of the day at latitude $32^\circ$, light spread over more ground, opposite seasons in the south, and what a different tilt would do.

Step 1 of 7 · Closest to the Sun

Step 1 of 7

Closest to the Sun

The first explanation most people reach for is distance: close to the Sun in summer, far in winter. The Earth’s orbit is indeed an ellipse, and the Scene draws it that way. But the ellipse is almost a circle, and the distance changes by only about $3\%$ ⁠.

Day of the year is in early July now, day $184$ ⁠, the height of summer in Israel. Predict when the Earth is closest to the Sun.

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Step 2 of 7

The tilted axis

If not the distance, then what? Move Day of the year right round the year and watch the Earth’s axis, the arrow through the poles.

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Step 3 of 7

A high Sun

It is the June solstice now. In the last Lesson you saw that the noon Sun stands at $h = 90^\circ - |\varphi - \delta|$ ⁠. At the June solstice $\delta$ ⁠ equals the axial tilt.

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Step 4 of 7

Long days

The length of the day changes with the season too. The Scene now shows the March equinox in Tel Aviv: the ring is cut almost exactly in two.

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Step 5 of 7

Light spread thin

A low Sun warms less, even when it shines just as brightly. When you choose Sunlight up close under Look inside after checking, you will see two equal beams of light: one falling straight down, and one at the latitude you chose. The yellow patch each lights on the ground is the same light, but under a low Sun it is smeared over a larger area, and each square metre gets less.

It is the December solstice now, day $354$ ⁠. Work it out before you open it.

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Step 6 of 7

The other hemisphere

Three things work together in summer: a high Sun, concentrated light, and long days. All of them come from the same tilt. And what is happening at the same time on the other side of the equator?

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Step 7 of 7

A different tilt

The Earth’s tilt is $23.4^\circ$ ⁠, but it need not be. Uranus is tilted almost $98^\circ$ ⁠, Jupiter only $3^\circ$ ⁠. In the Scene you can change it. Try $0^\circ$ ⁠ first: the dashed line sticks to the equator, and on every day of the year the day in Tel Aviv lasts about 12 h. Without a tilt there are no seasons.

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Interactive scene

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Each step sets up the Scene for you, and a control marked “fixed” stays put for that step.

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Practice · Predict, then look

The Earth’s orbit is an almost circular ellipse. When is the Earth closest to the Sun?

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Practice · Choose

So why is June summer in the northern hemisphere?

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Practice · Calculate

At the June solstice the Sun is overhead at $23.4^\circ$ ⁠ north, the Tropic of Cancer. How high does the noon Sun stand at latitude $32^\circ$ ⁠ north?

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Practice · Predict, then look

It is now the March equinox in Tel Aviv: about 12 h of daylight. Predict how long the day there lasts at the June solstice.

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Practice · Calculate

The December solstice, $32^\circ$ ⁠ north. How much sunlight does each square metre of level ground get at noon, as a per cent of what it would get with the Sun overhead? Write the number only.

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Practice · Predict, then look

The June solstice. In Tel Aviv the day is long, about 14 h. What happens on the same date in Sydney, at latitude $33.9^\circ$ ⁠ south?

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Practice · Reach the target

On our Earth the Sun is never overhead in Tel Aviv. Change Axial tilt until the noon Sun at the June solstice stands straight overhead at latitude $32^\circ$ ⁠ north, then press Check.