The Earth and the Solar System Cambridge IGCSE Combined Science revision
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In plain words
The Earth spins once a day, which gives us day and night, and goes round the Sun once a year. Because the Earth is tilted, different parts lean towards the Sun at different times of year: the seasons. Gravity is the invisible string that keeps planets, moons and comets in their orbits.
Three things to know
- The Earth spins on its axis once a day (day and night) and orbits the Sun once a year. Its tilted axis causes the seasons. The Moon orbits the Earth in about a month.
- The Solar System has one star, eight planets (four small and rocky, then four large and gaseous), dwarf planets, moons, asteroids and comets. Gravity keeps them in orbit.
- Orbital speed = 2 × π × orbital radius ÷ orbital period (v = 2πr/T).
Worked example
A planet orbits its star in a circle of radius 1.5 × 10¹¹ m once every 3.0 × 10⁷ s. Find its orbital speed.
- v = 2πr ÷ T.
- = 2 × π × 1.5 × 10¹¹ ÷ (3.0 × 10⁷).
- = 3.1 × 10⁴ m/s.
Tips and tricks
- The seasons are caused by the tilt, not by the Earth being nearer the Sun. That's the classic wrong answer.
- In v = 2πr ÷ T, the period T has to be in seconds.
It lands in your notebook with its questions as flashcards.
The Earth and the Solar System: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
How long does the Earth take to orbit the Sun?
One orbit is about 365 days.
What causes the seasons?
The tilt means each hemisphere leans towards the Sun for part of the year.
What keeps the planets in orbit around the Sun?
The Sun's gravitational pull acts towards it.
Which planets are closest to the Sun?
Mercury, Venus, Earth and Mars are small and rocky.
A moon orbits a planet in a circle of radius r with period T. What is its speed?
It travels the circumference, 2πr, in one period.
Where does a comet move fastest in its orbit?
The Sun's gravity is strongest there and has speeded it up.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 9 marks. Write your answers on paper, then check them.
The Earth and the Solar System
Cambridge IGCSE Combined Science 0653 · 9 marks · papermunch.org
Name ______________________________ Date ______________
Explain what causes day and night.[2]
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The Earth spins on its axis; the side facing the Sun has day and the other side has night.
Name the force that keeps planets in orbit, and state what provides it.[2]
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Gravity (gravitational attraction), from the Sun.
A satellite orbits at a radius of 7.0 × 10⁶ m and takes 6000 s for one orbit. Calculate its orbital speed.[3]
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7300 m/s (7.3 × 10³ m/s). v = 2π × 7.0 × 10⁶ ÷ 6000.
Describe one way the orbit of a comet differs from that of a planet.[2]
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A comet's orbit is a very elongated ellipse, so its distance from the Sun (and its speed) changes a great deal.
Answers: The Earth and the Solar System
- 1. The Earth spins on its axis; the side facing the Sun has day and the other side has night.
- 2. Gravity (gravitational attraction), from the Sun.
- 3. 7300 m/s (7.3 × 10³ m/s). v = 2π × 7.0 × 10⁶ ÷ 6000.
- 4. A comet's orbit is a very elongated ellipse, so its distance from the Sun (and its speed) changes a great deal.