Cambridge Lower Secondary CheckpointStage 6

Earth and Space: Earth in space

Science Stage 6 Chapter Notes

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Earth and Space: Earth in space
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In this chapter

  1. 1The Earth's Rotation: Day and Night
  2. 2The Earth's Orbit and The Seasons
  3. 3The Moon: Phases and Eclipses
  4. 4Gravity, Mass and Weight
  5. 5The Solar System and Orbits
  6. 6Artificial Satellites and their Orbits

1. The Earth's Rotation: Day and Night

The Earth spins on an imaginary line called its axis, which passes through the North and South poles. This spinning motion is called rotation. It takes the Earth approximately 24 hours to complete one full rotation. As the Earth rotates, different parts of its surface are exposed to the Sun's light. The side of the Earth facing the Sun experiences daylight, while the side facing away is in darkness, experiencing night. This constant rotation is what causes the daily cycle of day and night and makes the Sun appear to rise in the east and set in the west.

Key term

Axis of Rotation: An imaginary line passing through an object's centre around which it rotates.

Common pitfall

Confusing rotation (the Earth spinning on its axis to cause day and night) with revolution (the Earth orbiting the Sun to cause years).

Fun fact

The Earth's rotation is gradually slowing down at a rate of about 1.8 milliseconds per century. This means days are getting longer, but you won't notice it in your lifetime!

Worked example 13 marks

If it is 12:00 PM (noon) in London, explain why it is morning in New York, which is to the west.

  1. 1

    The Earth rotates on its axis from west to east.

  2. 2

    This means the Sun appears to move across the sky from east to west.

  3. 3

    Locations to the east of a point will experience sunrise and noon earlier.

  4. 4

    Since London is east of New York, the Sun reaches its highest point (noon) in London several hours before it does in New York.

  5. 5

    Therefore, when it is noon in London, the Earth has not yet rotated enough for New York to be directly under the Sun, so it is still morning there.

Recap

  • The Earth rotates on a tilted axis once every 24 hours.
  • This rotation causes the cycle of day and night.
  • The Sun appears to move across the sky because the Earth is spinning.
  • The direction of Earth's rotation is from west to east.

Quick check

  1. What term describes the spinning of the Earth on its axis?1 mark
  2. Approximately how long does one full rotation of the Earth take?1 mark

2. The Earth's Orbit and The Seasons

The seasons are not caused by the Earth's distance from the Sun. They are caused by the fact that the Earth's axis is tilted by 23.5 degrees. As the Earth completes its 365.25-day orbit (or revolution) around the Sun, this tilt remains pointed in the same direction in space. When a hemisphere is tilted towards the Sun, it receives more direct, concentrated sunlight, and the days are longer. This results in summer. When that same hemisphere is tilted away from the Sun, the sunlight is more spread out and less intense, and the days are shorter, resulting in winter. Spring and autumn (equinoxes) occur when both hemispheres receive roughly equal amounts of sunlight.

Key term

Axial Tilt: The angle between a planet's rotational axis and a line perpendicular to its orbital plane, which is the primary cause of seasons.

Examiner insight

Examiners award high marks for answers that clearly link the axial tilt to the concentration of solar rays and the length of the day, as these are the two key factors causing seasonal temperature changes.

Common pitfall

The most common misconception is believing that seasons are caused by the Earth being closer to or farther from the Sun in its elliptical orbit.

Worked example 14 marks

Explain why the UK (in the Northern Hemisphere) experiences summer in June, while Australia (in the Southern Hemisphere) experiences winter at the same time.

  1. 1

    The Earth's axis is tilted at an angle of 23.5 degrees relative to its orbit.

  2. 2

    In June, the Northern Hemisphere is tilted towards the Sun.

  3. 3

    This causes solar energy to be more concentrated on the Northern Hemisphere and leads to longer daylight hours, resulting in summer in the UK.

  4. 4

    Simultaneously, the Southern Hemisphere is tilted away from the Sun. The solar energy is more spread out, and daylight hours are shorter, resulting in winter in Australia.

Recap

  • The Earth revolves around the Sun in an orbit that takes approximately 365.25 days.
  • Seasons are caused by the Earth's constant 23.5° axial tilt, not its distance from the Sun.
  • Summer occurs in the hemisphere tilted towards the Sun, which receives more direct solar energy.
  • Winter occurs in the hemisphere tilted away from the Sun.
  • The longest and shortest days are called solstices, while equinoxes have nearly equal day and night.

Quick check

  1. What is the primary cause of the Earth's seasons?1 mark
  2. During the June solstice, which hemisphere is tilted towards the Sun?1 mark

3. The Moon: Phases and Eclipses

The Moon is Earth's only natural satellite, orbiting our planet roughly once every 27.3 days. The Moon does not produce its own light; we see it because it reflects sunlight. The 'phases' of the Moon are the different shapes we see as it orbits the Earth. These phases, from New Moon to Full Moon and back again, are due to our changing perspective of the Moon's sunlit half. Occasionally, the Sun, Earth, and Moon align perfectly, causing an eclipse. A solar eclipse occurs when the Moon passes between the Sun and Earth, casting a shadow on Earth (Sun-Moon-Earth). A lunar eclipse occurs when the Moon passes into Earth's shadow (Sun-Earth-Moon).

Key term

Eclipse: An astronomical event where one celestial body is temporarily obscured by passing into the shadow of another body or by having another body pass between it and the viewer.

Fun fact

We always see the same face of the Moon from Earth. This is because the Moon rotates on its axis in the same amount of time it takes to orbit the Earth, a phenomenon called 'tidal locking'.

Worked example 14 marks

Draw a simple, labelled diagram to show the alignment of the Sun, Earth, and Moon during a lunar eclipse and explain what is observed from Earth.

  1. 1

    Diagram should show the Sun, then the Earth, then the Moon in a straight line.

  2. 2

    Labels: Sun, Earth (casting a shadow), Moon (within the shadow).

  3. 3

    A lunar eclipse occurs when the Earth is positioned directly between the Sun and the Moon.

  4. 4

    The Earth blocks the sunlight from reaching the Moon, casting a shadow over it.

  5. 5

    From Earth, we observe the Full Moon dimming and often turning a reddish colour as it passes through the Earth's shadow.

Recap

  • The Moon is a natural satellite that orbits the Earth.
  • The phases of the Moon are due to the changing viewing angle of its sunlit side from Earth.
  • A solar eclipse is when the Moon's shadow falls on the Earth (Sun-Moon-Earth).
  • A lunar eclipse is when the Earth's shadow falls on the Moon (Sun-Earth-Moon).
  • Eclipses are rare because the Moon's orbit is tilted relative to Earth's orbit.

Quick check

  1. What is the alignment of the Sun, Earth and Moon during a New Moon phase?1 mark
  2. What is the difference between a natural satellite and an artificial satellite?2 marks

4. Gravity, Mass and Weight

Mass and weight are often confused, but they are different concepts. Mass is the amount of matter, or 'stuff', in an object. It is measured in kilograms (kg) and is an intrinsic property of an object that stays the same no matter where it is in the universe. Weight is the force of gravity acting on an object's mass. It is a force, so it is measured in Newtons (N). Your weight depends on the strength of the gravitational field you are in. The relationship is given by the equation: Weight = mass × gravitational field strength, or W = mg. On Earth, the gravitational field strength(g) is approximately 9.8 N/kg. On the Moon, g is only 1.6 N/kg, so you would weigh much less there, even though your mass remains unchanged.

Weight = mass × gravitational field strength

W = m × g

Key term

Gravitational Field Strength (g): The force per unit mass experienced by a small test mass placed at a point in a gravitational field, measured in Newtons per kilogram (N/kg).

Examiner insight

Examiners specifically look for the correct use of units (kg for mass, N for weight) and a clear statement that mass is constant while weight varies. Simply calculating a value is not enough without the correct units.

Common pitfall

Using the terms 'mass' and 'weight' interchangeably or providing the wrong units (e.g., 'weight in kg').

Worked example 14 marks

A rover has a mass of 900 kg. The gravitational field strength on Mars is 3.7 N/kg. Calculate:(a) the rover's weight on Earth (use g = 9.8 N/kg), and(b) the rover's mass and weight on Mars.

  1. 1

    (a) First, calculate the rover's weight on Earth.

  2. 2

    Formula: W = m × g

  3. 3

    Weight on Earth = 900 kg × 9.8 N/kg = 8820 N.

  4. 4

    (b) Now, consider the rover on Mars.

  5. 5

    The mass of the rover is constant, so its mass on Mars is still 900 kg.

  6. 6

    Weight on Mars = m × g_Mars

  7. 7

    Weight on Mars = 900 kg × 3.7 N/kg = 3330 N.

Recap

  • Mass is the amount of matter in an object, measured in kilograms (kg).
  • Weight is the force of gravity on an object, measured in Newtons (N).
  • Mass is constant everywhere, but weight changes with location.
  • The formula linking them is W = mg.
  • On Earth, g is approximately 9.8 N/kg.

Quick check

  1. An apple has a mass of 0.15 kg. What is its weight on Earth? (Take g = 10 N/kg)1 mark
  2. If an astronaut travels from the Earth to the Moon, what happens to their mass and their weight?2 marks

5. The Solar System and Orbits

Our Solar System is a gravitationally bound system comprising the Sun and the objects that orbit it. The Sun, a star, is at the centre and contains over 99.8% of the system's mass. The eight planets orbit the Sun in the following order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The force of gravity from the Sun keeps everything in orbit. The planets' orbits are not perfect circles but are slightly oval-shaped paths called ellipses. The closer a planet is to the Sun, the stronger the gravitational pull, and the faster it must travel to maintain a stable orbit. This is why inner planets like Mercury have much shorter years than outer planets like Neptune. The Solar System also contains dwarf planets (like Pluto), asteroids (mostly between Mars and Jupiter), and comets.

Key term

Orbit: The curved, elliptical path of a celestial object or spacecraft around a star, planet, or moon, held in place by gravity.

Examiner insight

Students who can explain *why* orbital speed changes with distance (linking it to the strength of gravity) score higher than those who just state that inner planets move faster.

Worked example 14 marks

Mercury takes only 88 Earth days to orbit the Sun, while Mars takes 687 Earth days. Using principles of gravity, explain this difference.

  1. 1

    The force of gravity provides the centripetal force needed to keep a planet in orbit.

  2. 2

    Mercury is much closer to the Sun than Mars is.

  3. 3

    Therefore, the Sun's gravitational pull on Mercury is significantly stronger than on Mars.

  4. 4

    To maintain a stable orbit and not fall into the Sun, Mercury must travel at a much higher orbital speed.

  5. 5

    The combination of a higher speed and a much shorter orbital path means Mercury completes its orbit in far less time than Mars.

Recap

  • The Solar System consists of the Sun, eight planets, and other celestial bodies.
  • The planets, in order, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
  • The force of gravity from the Sun keeps all objects in elliptical orbits.
  • Planets closer to the Sun orbit faster and have shorter years.
  • The asteroid belt is located between the orbits of Mars and Jupiter.

Quick check

  1. Name the four gas giants in our Solar System.2 marks
  2. What force is responsible for the formation and structure of the Solar System?1 mark

6. Artificial Satellites and their Orbits

Artificial satellites are human-made objects intentionally placed into orbit around a celestial body, usually the Earth. To stay in orbit, a satellite must travel at a very high speed. It is continuously falling towards the Earth due to gravity, but its high horizontal velocity means it constantly 'misses' the ground, following the curve of the planet. There are two main types of orbit. A Low Earth Orbit (LEO) is relatively close to Earth (200-2000 km altitude). Satellites in LEO, like the International Space Station (ISS) and spy satellites, orbit the Earth very quickly, typically in about 90 minutes. A Geostationary Orbit (GEO) is much higher (35,786 km altitude) directly above the equator. In this orbit, a satellite takes exactly 24 hours to circle the Earth, matching the planet's rotation. This makes it appear stationary from the ground, which is ideal for communications and weather satellites.

Key term

Geostationary Orbit: A high Earth orbit that allows a satellite to match Earth's rotation, causing it to appear stationary from a fixed point on the ground.

Fun fact

There are over 6,000 artificial satellites orbiting Earth, but only about 40% of them are currently operational. The rest are 'space junk'.

Worked example 13 marks

A company wants to launch a new satellite for its global positioning system (GPS). This requires a constellation of satellites that cover the entire Earth's surface. Would a geostationary orbit be suitable? Explain your answer.

  1. 1

    No, a geostationary orbit would not be suitable for a single GPS satellite aiming for global coverage.

  2. 2

    A geostationary satellite remains fixed over one point on the equator.

  3. 3

    While this is good for broadcasting to a specific region, it cannot provide positioning data for the entire globe.

  4. 4

    GPS systems use a constellation of multiple satellites in medium Earth orbits (a type of non-geostationary orbit) so that from any point on Earth, at least four satellites are always visible in the sky.

Recap

  • Artificial satellites are kept in orbit by balancing their high forward speed with the force of Earth's gravity.
  • Low Earth Orbit (LEO) is used for observation and has a short orbital period (e.g., 90 minutes).
  • Geostationary Orbit (GEO) has a 24-hour period, making the satellite appear stationary.
  • Geostationary orbits are ideal for communication and weather monitoring.
  • Different orbits are chosen based on the satellite's specific mission.

Quick check

  1. What is the orbital period of a geostationary satellite?1 mark
  2. State two common uses of artificial satellites.2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the terms 'rotation' and 'revolution' as they apply to the Earth in space, and state the time period for each.4 marks
  2. An astronaut has a total weight of 735 N on Earth. The gravitational field strength on Mars is 3.7 N/kg. Calculate the astronaut's mass, and their weight on Mars. (Take g on Earth = 9.8 N/kg).4 marks
  3. A student claims that it is summer in the Northern Hemisphere because the Earth's orbit brings it closer to the Sun. Explain why this student is incorrect and provide the correct scientific explanation for the seasons.5 marks
  4. Compare and contrast a solar eclipse and a lunar eclipse. You may use simple, labelled diagrams to support your answer.6 marks
  5. Describe the key differences between a Low Earth Orbit (LEO) and a Geostationary Orbit (GEO). For each type of orbit, give one example of a satellite that uses it and explain why that orbit is suitable.5 marks
  6. List the eight planets of the solar system in the correct order starting from the Sun. Classify each one as either a 'rocky planet' or a 'gas giant'.4 marks
  7. Explain why a satellite in a stable orbit around the Earth does not fall to the ground, even though it is constantly being pulled by gravity.3 marks
  8. Explain why we see different phases of the Moon over the course of a month.3 marks
  9. What is the difference between mass and weight? State the SI unit for each quantity.3 marks
  10. The planet Jupiter has a much shorter day (about 10 hours) but a much longer year (about 12 Earth years) than Earth. Explain these two facts.4 marks

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