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Lessons / Climate

Climate · 5 min · 5 steps

Seasons: tilt, not distance

Plays here

Goal. Explain the seasons with the tilt of the Earth's axis, using day length and the height of the noon Sun.

Steps

  1. Predict

    When is the Earth closest to the Sun?

    • In northern summer, around July
    • In northern winter, in early January
    • The distance never changes
    Show the answer

    In northern winter, in early January. In early January, about 147 million km away, in the middle of northern winter.

    The shaded half of the globe is in night, and the sun symbol marks where the Sun is overhead right now. If distance made the seasons, the whole Earth would have summer at the same time.

  2. Around 21 June the north leans towards the Sun. The noon Sun is overhead on the Tropic of Cancer, which crosses Taiwan, and inside the Arctic Circle the Sun does not set at all.

  3. Around 21 December the south leans towards the Sun. Now the Arctic has no sunrise, Australia has summer, and the noon Sun over Taiwan is low in the south.

  4. Taipei's card shows the length of the day and the height of the noon Sun. On the June solstice the day lasts about 13 hours 40 minutes and the Sun climbs to 88°.

  5. On the December solstice the day in Taipei is about three hours shorter and the noon Sun reaches only about 42°. Low sunshine is spread over more ground, so each square metre gets less energy.

The common mistake

Myth

Summer comes when the Earth is closest to the Sun.

Fact. The Earth is closest to the Sun in early January, in the middle of northern winter. The seasons come from the tilt of the axis, which changes how high the Sun climbs and how long the days last, and the two hemispheres have opposite seasons.

Check yourself

Question 1

What causes the seasons?

  • The Earth's changing distance from the Sun
  • The tilt of the Earth's axis
  • The Sun burning hotter in summer
Show the answer

The tilt of the Earth's axis. The tilt changes the height of the noon Sun and the length of the day through the year. Distance changes only a little and in the wrong direction for the north.

Question 2

On about 21 June, where is the noon Sun directly overhead?

  • On the equator
  • On the Tropic of Cancer, which crosses Taiwan
  • At the North Pole
Show the answer

On the Tropic of Cancer, which crosses Taiwan. At the June solstice the Sun is overhead at 23.4° N, the Tropic of Cancer, which crosses Chiayi and Hualien.

Question 3

Why does low winter sunshine warm the ground less?

  • The Sun gives out less light in winter
  • The same beam of light is spread over more ground
  • The Moon blocks some of it
Show the answer

The same beam of light is spread over more ground. A low Sun hits the ground at a slant, so each square metre receives less energy, and the short days add up to fewer hours of sunshine.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
Id-Ⅳ-2 · Id-Ⅳ-3 · EId-Vc-1
US Next Generation Science Standards
MS-ESS1-1
England national curriculum
KS3 Physics: the seasons and the Earth's tilt, day length at different times of year