Seasons & Day/Night

Earth's Rotation and Revolution

Earth has two important movements:

Rotation: Earth spins on its own axis once every 24 hours. This gives us day and night — the side facing the Sun experiences day, the side facing away experiences night.
Revolution: Earth orbits (revolves) around the Sun once every 365.25 days (one year). The extra quarter day is why we add a leap day (29 February) every four years.

Common misconception: Seasons are NOT caused by Earth being closer to or farther from the Sun. In fact, Earth is slightly closer to the Sun in January (Southern Hemisphere summer). Seasons are caused by Earth's axial tilt.

Earth's Axial Tilt

Earth's axis is tilted at 23.5° relative to its orbital plane around the Sun. This tilt always points in the same direction in space (toward Polaris, the North Star). As Earth orbits the Sun, different hemispheres are tilted toward or away from the Sun at different times of year.

When a hemisphere is tilted toward the Sun: the Sun is higher in the sky, days are longer, sunlight is more concentrated → summer
When a hemisphere is tilted away from the Sun: the Sun is lower in the sky, days are shorter, sunlight is more spread out → winter

The Southern and Northern Hemispheres always have opposite seasons. When it is summer in South Africa (December), it is winter in Europe.

Solstices and Equinoxes (Southern Hemisphere)

Event	Approx. date	What happens	Season in S.A.
Summer Solstice	~21 December	Southern Hemisphere tilted most toward Sun; longest day	Start of Summer
Autumn Equinox	~21 March	Neither hemisphere tilted toward Sun; day = night (~12 h each)	Start of Autumn
Winter Solstice	~21 June	Southern Hemisphere tilted most away from Sun; shortest day	Start of Winter
Spring Equinox	~23 September	Neither hemisphere tilted toward Sun; day = night (~12 h each)	Start of Spring

Solstice comes from the Latin "sol" (sun) + "sistere" (to stand still) — the Sun appears to reach its highest or lowest point and then reverse. Equinox means "equal night" — day and night are each approximately 12 hours everywhere on Earth.

Why Daylight Hours Change

Because Earth's axis is tilted, the circle of illumination (the boundary between day and night) does not pass through the poles. In summer, a location in the Southern Hemisphere spends more of its daily rotation on the sunlit side → more daylight hours. In winter, the opposite is true.

At the equator, day and night are always close to 12 hours each, year-round.
At the poles, there can be continuous day (midnight sun) or continuous night in extreme seasons.
In South Africa (mid-latitudes), daylight ranges from about 10 hours in winter to about 14 hours in summer.

Why Seasons Occur: Two Key Reasons

When a hemisphere is tilted toward the Sun, two things happen that make it warmer:

Higher sun angle: Sunlight strikes the ground more directly (less spread out), so energy per square metre is greater.
Longer days: More hours of daylight means more total energy received each day.

Time Zones

Because Earth is a sphere rotating 360° in 24 hours, it rotates 15° per hour. The world is divided into approximately 24 time zones, each roughly 15° of longitude wide. Countries to the east of you experience sunrise earlier. South Africa uses SAST (South Africa Standard Time) = UTC+2 — two hours ahead of Greenwich Mean Time.

South Africa does not use daylight saving time — our clocks stay the same all year round.

Answer the following questions in your exercise book. Use diagrams where they help explain your answer.

Question 1 · (2 marks)

Distinguish between Earth's rotation and its revolution. Give the period (time taken) for each.

Question 2 · (3 marks)

Explain why Earth has seasons. In your answer: (a) state the angle of Earth's axial tilt, (b) explain what happens when the Southern Hemisphere is tilted toward the Sun, and (c) explain why distance from the Sun is NOT the main cause of seasons.

Question 3 · (4 marks)

Draw a diagram showing Earth at four positions in its orbit around the Sun. Label: summer solstice, winter solstice, spring equinox, and autumn equinox for South Africa. Include the approximate dates for each event.

Question 4 · (3 marks)

Explain why daylight hours are longer in summer than in winter. In your answer, refer to Earth's axial tilt and the circle of illumination. Approximately how many hours of daylight does South Africa receive in summer versus winter?

Question 5 · (2 marks)

What is an equinox? On what dates do the equinoxes occur in South Africa, and what do they mark in terms of seasons?

Question 6 · (2 marks)

Explain what a time zone is and why they exist. What is South Africa's time zone (SAST) relative to UTC?

Question 7 · (4 marks)

The table below shows approximate sunrise and sunset times for a town in South Africa on three dates.

Date	Sunrise	Sunset
21 June	07:50	17:50
23 September	06:00	18:00
21 December	05:30	19:30

(a) Calculate the number of daylight hours for each of the three dates. (2)
(b) Match each date to the event it represents: summer solstice, spring/autumn equinox, or winter solstice. (1)
(c) Calculate the difference in daylight hours between 21 December and 21 June. (1)

Earth's Rotation and Revolution

Earth's Axial Tilt

Solstices and Equinoxes (Southern Hemisphere)

Why Daylight Hours Change

Why Seasons Occur: Two Key Reasons

Time Zones

Seasons & Day/Night Simulator