The Earth, the Moon, Day/Night and Seasons

Physics· 6.1.1· 15 min read

1. Core: Earth and Moon Motions and the Moon's Phases★☆☆☆☆⏱ 5 min

📘 Definition

Rotation

The spinning of a celestial body around its own internal axis. The Earth completes one full rotation every 24 hours.

Example:

Earth's rotation on its axis causes the apparent movement of the Sun across the sky each day.

The Earth has two key motions: rotation on its axis (24 hour period) and revolution in its orbit around the Sun (365.25 day period, one Earth year). The Moon orbits (revolves around) the Earth once about every 28 days (roughly one month).

📐 Worked Example

State two differences between the Earth's rotation and its revolution around the Sun.

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1. Recall definitions: Rotation is spinning on the Earth's internal axis; revolution is orbital motion around the Sun.
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1. First difference: Rotation takes 24 hours to complete, while revolution takes ~365 days to complete.
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1. Second difference: Rotation causes the day/night cycle, while revolution contributes to seasonal cycles.

The Moon's phases. The Moon does not produce its own light — we see it because it reflects sunlight, and the Sun always lights up one half of the Moon. As the Moon takes about one month (~28 days) to orbit the Earth, the relative positions of the Sun, Earth and Moon change, so from Earth we see different amounts of the Moon's sunlit half. This produces the repeating cycle of phases, and because it is caused by the Moon's orbit, the full cycle of phases also takes about one month.

New Moon: the Moon is roughly between the Earth and the Sun, so its sunlit half faces away from us and the Moon looks dark.
Full Moon: the Earth is roughly between the Sun and the Moon, so we see the whole of the Moon's sunlit half.
First and last quarter: the Sun, Earth and Moon make a right angle, so we see exactly half of the sunlit face.
Waxing means the lit part we see is growing (New Moon towards Full Moon); waning means it is shrinking (Full Moon towards New Moon).

📐 Worked Example

Explain why the Moon shows a repeating cycle of phases (such as New Moon to Full Moon and back) that takes about one month.

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1. We see the Moon only because it reflects sunlight; the Sun always lights up one half of the Moon.
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1. As the Moon orbits the Earth, the relative positions of the Sun, Earth and Moon change, so from Earth we see different fractions of the Moon's sunlit half.
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1. The Moon takes about one month (~28 days) to complete one orbit of the Earth, so the full cycle of phases also repeats about once a month.

Exam tip:

Always specify the reference body when describing orbital motion (e.g. "revolution around the Sun", not just "revolution"). When explaining the Moon's phases, link them to the changing Sun-Earth-Moon positions as the Moon orbits over about a month, and not to the Earth's shadow (that would be a lunar eclipse).

2. Core: Cause of the Day/Night Cycle★☆☆☆☆⏱ 3 min

The day/night cycle is caused exclusively by the Earth's constant rotation on its axis. As the Earth spins, any point on its surface faces the Sun for roughly half the 24 hour rotation period (daytime) and faces away from the Sun for the other half (nighttime).

📐 Worked Example

A student claims that day and night are caused by the Earth orbiting the Sun. Explain why this claim is incorrect.

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1. Identify the correct cause of day/night: The cycle is produced by the Earth's rotation on its axis, not its revolution around the Sun.
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1. Explain the error in the student's claim: If day/night was caused by revolution around the Sun, one full day/night cycle would take 365 days, not 24 hours.
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1. Confirm the role of revolution: The Earth's orbit around the Sun contributes to seasonal cycles, not daily day/night cycles.

3. Core: Cause of Seasonal Cycles★★☆☆☆⏱ 4 min

📘 Definition

Axial Tilt

The angle between a planet's rotational axis and the perpendicular to its orbital plane. The Earth's axis is tilted at a fixed angle of 23.5° relative to its orbit around the Sun.

Seasons are caused by the combination of the Earth's fixed 23.5° axial tilt and its revolution around the Sun. When a hemisphere is tilted towards the Sun, it receives more direct sunlight, longer daylight hours, and warmer temperatures (summer). When tilted away from the Sun, it receives less direct sunlight, shorter days, and cooler temperatures (winter). Equinoxes occur twice per year, when the axis is tilted neither towards nor away from the Sun, so day and night are equal length globally.

📐 Worked Example

Explain why Australia, located in the southern hemisphere, experiences summer in December.

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1. Recall the Earth's fixed axial tilt of 23.5° relative to its orbital plane around the Sun.
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1. State the Earth's position in December: The southern hemisphere is tilted directly towards the Sun in December, while the northern hemisphere is tilted away.
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1. Explain the effect: The southern hemisphere receives more direct sunlight and longer daylight hours in December, leading to summer temperatures.

Exam tip:

Never reference the Earth's distance from the Sun when explaining seasons in exams; this is a common wrong answer that will lose marks.

4. Extended: Orbital Speed Calculations★★★☆☆Extended only⏱ 5 min

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📘 Definition

Orbital Speed

$v$

The linear speed of a celestial body as it moves along its circular orbital path, calculated using the formula $v = \frac{2 π r}{T}$ , where $r$ is the radius of the orbit, and $T$ is the time taken for one full orbit (the orbital period).

For Extended tier candidates, you will be expected to calculate the orbital speed of the Earth, Moon, or other orbiting bodies. Always ensure units are consistent before substituting values: if speed is requested in m/s, convert the orbital period to seconds and the orbital radius to metres first.

📐 Worked Example

The Moon orbits the Earth at an average orbital radius of $3.8 \times 1 0^{8}$ m, with an orbital period of 28 days. Calculate the orbital speed of the Moon in m/s, to 2 significant figures.

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1. Convert the orbital period from days to seconds: $28 days = 28 \times 24 \times 60 \times 60 = 2.4192 \times 1 0^{6} s$
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1. Write the orbital speed formula:
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$v = \frac{2\pi r}{T}$
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1. Substitute the known values:
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$v = \frac{2 \times \pi \times 3.8 \times 10^8}{2.4192 \times 10^6}$
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1. Calculate the result: $v \approx 990 m/s$ (2 significant figures)

5. Common Pitfalls

Wrong move:

Claiming seasons are caused by the Earth being closer to the Sun in summer

Why:

The Earth's nearly circular orbit leads to negligible distance variation from the Sun; seasons are caused exclusively by axial tilt

Correct move:

Explain seasons by referencing which hemisphere is tilted towards the Sun at a given point in the Earth's orbit

Wrong move:

Stating the Moon does not rotate on its axis

Why:

The Moon rotates on its axis at exactly the same rate as it orbits the Earth, so only one side is visible from Earth

Correct move:

State the Moon's rotational period equals its orbital period around the Earth, resulting in synchronous rotation

Wrong move:

Using orbital period in days when calculating orbital speed in m/s

Why:

Units must be consistent; days are not SI units for time, so calculations will be incorrect

Correct move:

Convert orbital period from days/hours to seconds before substituting into the $v = 2 π r / T$ formula

Wrong move:

Confusing rotation and revolution when explaining day/night and seasons

Why:

Rotation causes day/night, while revolution combined with tilt causes seasons; mixing these up loses exam marks

Correct move:

Explicitly link rotation to day/night cycles, and revolution + axial tilt to seasonal cycles

Wrong move:

Claiming equinoxes occur when the Earth's axis is not tilted

Why:

The Earth's axis is permanently tilted at 23.5°; equinoxes occur when the tilt is oriented perpendicular to the Earth-Sun line

Correct move:

Explain equinoxes as the point in orbit where both hemispheres receive equal hours of daylight and darkness

6. Quick Reference Cheatsheet

Concept	Core Fact	Extended Fact (if applicable)
Day/Night Cycle	Caused by Earth's rotation on its axis every 24 hours	N/A
Seasons	Caused by 23.5° axial tilt + Earth's revolution around Sun every 365.25 days	N/A
Moon Orbit	Orbits Earth every ~28 days; same face always faces Earth	N/A
Moon Phases	Caused by seeing different amounts of the Moon's sunlit half as it orbits Earth; full cycle takes ~1 month	N/A
Orbital Speed	N/A	Calculated using $v = 2 π r / T$ , where $r$ =orbit radius, $T$ =period in seconds

7. Frequently Asked

Are seasons caused by the Earth being closer to the Sun in summer?

No. The Earth's nearly circular orbit leads to negligible distance variation from the Sun. Seasons are caused by the 23.5° tilt of the Earth's axis: a hemisphere tilted towards the Sun receives more direct sunlight and longer days, leading to summer.

Why do we only see one side of the Moon from Earth?

The Moon's rotational period (time to spin once on its axis) exactly matches its orbital period around the Earth (~28 days). This synchronous rotation means the same face of the Moon always faces Earth.

What is the difference between a solstice and an equinox?

A solstice occurs when one hemisphere is tilted maximally towards/away from the Sun, leading to the longest/shortest day of the year. An equinox occurs when the axis is tilted perpendicular to the Earth-Sun line, so day and night are equal length globally.

What causes the phases of the Moon?

We see the Moon because it reflects sunlight, and the Sun lights up one half of it. As the Moon orbits the Earth over about a month, the Sun-Earth-Moon positions change, so from Earth we see different amounts of the sunlit half - giving the cycle from new Moon to full Moon and back. The phases are not caused by the Earth's shadow (that would be a lunar eclipse).

Going deeper

Syllabus DocumentCIE IGCSE Physics 0625 2026-2028 SyllabusOfficial specification for this subtopic

What's Next

Now that you have mastered the foundational motion of the Earth and Moon, and the origins of day/night and seasonal cycles, you are ready to progress to more advanced Space Physics topics in the CIE IGCSE 0625 syllabus. Next, you will learn about the structure of the Solar System, including planets, asteroids, comets, and the Sun, followed by the life cycle of stars and the structure of the universe. If you are taking the Extended tier, make sure to practice orbital speed calculations regularly, as these are common high-mark questions in Paper 2 and Paper 4 exams. Mastering this orbital motion content will also help you understand artificial satellite behaviour, a later subtopic in the Space Physics unit.