IGCSE Physics: Earth and the Solar System Practice Questions
The Earth rotates once every 24 hours, producing day and night, and orbits the Sun once every 365 days. The eight planets orbit the Sun in elliptical paths held by the Sun's gravitational attraction, with the nearer planets moving faster.
Sub-topic 6.1 of Cambridge IGCSE Physics 0625 mixes recall with one orbital speed calculation. The most common error is confusing rotation with orbit, which changes every answer that follows. The questions below separate them and cover the calculation.
What you need to know for Earth and the Solar System
- Rotation and orbitThe Earth rotates on its own axis once in approximately 24 hours, which causes day and night. It orbits the Sun once in approximately 365 days, which defines the year.
- The MoonThe Moon orbits the Earth in approximately one month. It is not a light source. We see it because it reflects light from the Sun.
- Order of the planetsMercury, Venus, Earth, Mars, then the asteroid belt, then Jupiter, Saturn, Uranus and Neptune.
- Rocky and gas planetsThe four inner planets are small and rocky. The four outer planets are much larger and made mainly of gases, having formed further from the Sun where lighter material could be retained.
- Orbital speedv = 2 pi r divided by T, where r is the average orbital radius and T is the orbital period. Planets closer to the Sun have shorter periods and move faster.
- Why orbits are maintainedThe gravitational attraction of the Sun provides the force that continually changes the direction of a planet's velocity, keeping it in a closed elliptical orbit rather than travelling in a straight line.
IGCSE Physics Earth and the Solar System questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Explain the difference between the Earth's rotation and the Earth's orbit, and state what each one causes.
Show the worked answer
- Rotation is the spinning of the Earth about its own axis, which passes through the north and south poles.
- One complete rotation takes approximately 24 hours. The side facing the Sun experiences day and the side facing away experiences night.
- The orbit is the path the Earth follows around the Sun, which is very nearly circular.
- One complete orbit takes approximately 365 days, and this period defines one year.
A planet has an average orbital radius of 1.5 x 1011 m and an orbital period of 3.2 x 107 s. Calculate its average orbital speed.
Show the worked answer
- The planet travels one full circumference in one orbital period, so use v = 2 pi r divided by T.
- Circumference = 2 x pi x 1.5 x 1011 = 9.42 x 1011 m.
- v = 9.42 x 1011 divided by 3.2 x 107.
- v = 2.95 x 104 m/s, which is 2.9 x 104 m/s to 2 significant figures, about 29 km/s.
Explain why the planets closest to the Sun have the shortest orbital periods.
Show the worked answer
- A planet closer to the Sun has a smaller orbital radius, so the circumference of its orbit is shorter and there is less distance to cover.
- The gravitational attraction of the Sun is stronger at smaller distances.
- A stronger attractive force is needed to maintain a faster orbit at that radius, so the closer planets move at greater orbital speeds.
- A shorter path travelled at a higher speed means a much shorter orbital period. Mercury takes about 88 days while Neptune takes about 165 years.
Explain why we can see the Moon and the planets even though they do not produce their own light.
Show the worked answer
- The Sun is a star and produces its own light through nuclear reactions in its core.
- The Moon and the planets are not light sources. They emit no visible light of their own.
- Light from the Sun travels outwards and falls on their surfaces.
- Some of that light is reflected, and the portion reflected towards the Earth enters our eyes, so we see them. This is also why the Moon shows phases, since we only see the illuminated part facing us.
Common mistakes in this topic
- Confusing rotation with orbit.
- Using the orbital radius instead of the circumference in a speed calculation.
- Getting the order of the planets wrong, particularly Uranus and Neptune.
- Saying gravity pulls a planet along its orbit. Gravity acts towards the Sun and changes the direction of the velocity.
- Describing the Moon as a light source.
Exam tips
- Write out the eight planets in order and check where the asteroid belt sits, between Mars and Jupiter.
- For any orbital speed question, the distance is always the full circumference, 2 pi r.
- Work in standard form throughout astronomy calculations to avoid losing zeros.
- Link orbital radius to both distance and gravitational force strength when explaining periods.
- Remember that the inner planets are small and rocky and the outer ones large and gaseous, and why.
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Earth and the Solar System FAQs
What causes day and night?
The rotation of the Earth about its own axis, which takes approximately 24 hours. The side of the Earth facing the Sun experiences day while the side facing away experiences night. Day and night are not caused by the Earth's orbit around the Sun, which takes a year.
How do I calculate the orbital speed of a planet?
Divide the circumference of the orbit by the orbital period, using v = 2 pi r divided by T. The distance travelled in one orbit is the whole circumference, not the radius, which is the most common error in this calculation.
Why do inner planets orbit faster than outer planets?
They have a smaller orbital radius, so their orbital path is shorter, and the Sun's gravitational attraction is stronger at that distance. Both factors mean a shorter distance travelled at a higher speed, giving a much shorter orbital period.
Why can we see the planets if they produce no light?
The Sun produces light through nuclear reactions in its core. That light travels outwards and falls on the planets and the Moon, which reflect some of it. The portion reflected towards Earth is what we see, which is also why the Moon shows phases.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.