The Earth, Moon and the Solar System
The structure of our Solar System, what causes day, night and the seasons, and why planets stay in orbit at all.
1. The Solar System
The Solar System consists of the Sun at the centre, with eight planets, their moons, plus asteroids, comets and dwarf planets orbiting it.
The order of the planets, from the Sun:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
A mnemonic: My Very Easy Method Just Speeds Up Naming.
Knowing the order matters — confusion about it was a recorded error, and several questions depend on which planets are inner and which outer.
| Inner (rocky) planets | Outer (gas giants) | |
|---|---|---|
| Which | Mercury, Venus, Earth, Mars | Jupiter, Saturn, Uranus, Neptune |
| Made of | rock and metal | gas and ice |
| Size | smaller | much larger |
| Orbit time | shorter | much longer |
Between Mars and Jupiter lies the asteroid belt.
The Sun is a star — it produces its energy by nuclear fusion of hydrogen into helium, and it contains most of the Solar System’s mass.
Planets do not produce their own light. We see them because they reflect sunlight. Stars produce their own light by fusion; planets do not.
2. Orbits and gravity
Planets stay in orbit because of the gravitational attraction between them and the Sun.
Gravity provides the force that keeps a planet moving in its (near) circular path — without it, a planet would travel in a straight line.
Orbital speed and distance:
Planets closer to the Sun orbit FASTER and have shorter years.
Mercury takes 88 days to orbit; Neptune takes about 165 years. The Sun’s gravitational pull is stronger closer in, so those planets must move faster to stay in orbit.
Calculating orbital speed:
average orbital speed = 2πr / T
where r is the orbital radius and T the time for one complete orbit — the circumference divided by the period.
Make sure r and T are in consistent units, and convert the period into seconds if you want the speed in m/s.
Orbits are slightly elliptical, not perfect circles — a comet’s orbit is highly elliptical, which is why comets speed up dramatically near the Sun (where gravity is strongest) and slow far away.
3. Day and night
Day and night are caused by the Earth’s ROTATION on its own axis, once every 24 hours.
The half of the Earth facing the Sun has day; the half facing away has night.
Rotation causes day and night. ORBIT causes the year. These are different motions and get confused: the Earth rotates on its axis in 24 hours and orbits the Sun in 365¼ days.
4. The seasons
The seasons are caused by the TILT of the Earth’s axis (about 23.5°) as it orbits the Sun.
In summer for a hemisphere, it is tilted towards the Sun, so:
- The Sun appears higher in the sky
- Sunlight strikes the surface more directly, concentrating energy over a smaller area
- Days are longer
In winter, that hemisphere is tilted away, so the light is more spread out and days are shorter.
The seasons are NOT caused by the Earth being closer to the Sun. This is the classic misconception. The Earth’s distance from the Sun barely changes — and the northern and southern hemispheres have opposite seasons at the same time, which distance alone could never explain.
5. The Moon
The Moon orbits the Earth in about 27–28 days, held by the Earth’s gravity.
The Moon does not produce its own light — it reflects sunlight.
Phases of the Moon: as the Moon orbits, we see different fractions of its sunlit half, producing the cycle new moon → crescent → half → gibbous → full and back.
Eclipses:
Solar eclipse — the Moon passes between the Sun and the Earth, blocking sunlight. Lunar eclipse — the Earth passes between the Sun and the Moon, so the Earth’s shadow falls on the Moon.
Tides are caused mainly by the Moon’s gravitational pull on the oceans.
6. Distances
A light year is the distance light travels in one year — about 9.5 × 10¹⁵ m.
Within the Solar System, distances are more often given in astronomical units (AU) — the average Earth–Sun distance.
Light takes about 8 minutes to reach us from the Sun, and hours to reach the outer planets — a useful sense of scale.
7. Mistakes that cost marks
Getting the planet order wrong.
Saying planets produce their own light.
Confusing rotation with orbit as the cause of day/night or the year.
Saying seasons are caused by distance from the Sun.
Saying distant planets orbit faster — they orbit slower.
Forgetting gravity as the force maintaining an orbit.
Mixing up solar and lunar eclipses.
Inconsistent units in an orbital speed calculation.
Frequently asked questions
What is the order of the planets? Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
What keeps planets in orbit? The gravitational attraction of the Sun.
Which planets orbit fastest? Those closest to the Sun, where gravity is strongest.
How do I calculate orbital speed? 2πr / T — circumference divided by the orbital period.
What causes day and night? The Earth’s rotation on its axis, once every 24 hours.
What causes the seasons? The tilt of the Earth’s axis as it orbits — not its distance from the Sun.
Why isn’t distance the cause of seasons? Because the two hemispheres have opposite seasons at the same time.
Does the Moon produce light? No — it reflects sunlight.
What causes a solar eclipse? The Moon passing between the Sun and the Earth.
What causes the tides? Mainly the Moon’s gravitational pull on the oceans.
Quick revision checklist
- I know the order of the planets and a mnemonic
- I can distinguish inner rocky from outer gas planets
- I know the Sun is a star powered by fusion
- I know planets reflect light rather than producing it
- I know gravity keeps planets in orbit
- I know closer planets orbit faster
- I can calculate orbital speed with 2πr/T
- I know rotation causes day and night
- I know axial tilt causes the seasons
- I can explain why distance does not cause the seasons
- I know the Moon’s orbit period and that it reflects light
- I can explain the Moon’s phases
- I can distinguish solar and lunar eclipses
- I know a light year is a distance
These notes cover the Earth, Moon and the Solar System in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons. This was one of the least-covered subtopics in that set, so the page follows the syllabus closely rather than being padded. Always check the current syllabus for your own exam series.
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