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Cambridge IGCSE0625

The Earth and the Solar System

Physics 0625 Chapter Notes

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The Earth and the Solar System - The EarthThe Earth and the Solar System - The Solar System
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In this chapter

  1. 1Rotation: day and night
  2. 2Orbit: the year and the seasons
  3. 3The Moon and its phases
  4. 4Average orbital speed
  5. 5Using the equation: period in seconds
  6. 6Orbit above the Earth's surface
  7. 7What is in the Solar System
  8. 8Rocky planets and gas giants
  9. 9Gravitational field strength
  10. 10Light travel time
  11. 11Why the planets orbit the Sun
  12. 12Elliptical orbits
  13. 13Planetary data and distance from the Sun
  14. 14Speed in an elliptical orbit

1. Rotation: day and night

The Earth is a planet. It rotates on its axis once in approximately 24 hours. The axis is tilted.

The Sun appears to move across the sky each day because the Earth rotates. This gives the periodic cycle of day and night.

Examiner insight

“Many candidates had a clear understanding that the Earth spins on its axis, but fewer stated that this takes 24 hours.” “Others were convinced that the motion of the Sun around the Earth was the cause of day and night.”

2. Orbit: the year and the seasons

The Earth orbits the Sun once in approximately 365 days. Its orbit is almost circular, an ellipse.

The Earth's axis is tilted as it orbits. Through the year, a given country receives the Sun's rays at different angles and for a different number of hours per day. This explains the periodic seasons.

Examiner insight

“A very common error was to state that winter was caused by an increase in the distance between the Earth and the Sun.” “Few were able to express that the rays from the Sun strike the country at different angles during the year or that the country received rays from the Sun for a different number of hours per day.”

Worked example 13 marks

Describe the motion of the planet Earth. In your answer, include the values for any times of rotation or orbit that you describe.

  1. 1

    the Earth rotates / spins on its axis [1]

  2. 2

    once every 24 hours [1]

  3. 3

    it takes 365 days to orbit the Sun [1]

3. The Moon and its phases

The Moon orbits the Earth once in approximately one month.

The Moon's phases repeat as it orbits. This gives the periodic cycle of phases, about once a month.

Examiner insight

“Some candidates confused this question with the length of a day on Earth.”

Worked example 11 mark

State the approximate time taken for the Moon to orbit the Earth. Include the unit.

  1. 1

    1 month [1]

4. Average orbital speed

Average orbital speed v is defined from the equation below. r is the average radius of the orbit. T is the orbital period: the time taken to complete one orbit.

The speed is an average value because the orbit is not exactly circular, so r varies.

v = (2π r)/T

Examiner insight

“Some candidates described r as the distance from the Sun, instead of defining r as the orbital radius.” “Answers defining r as radius and T as time were insufficient and those defining r as the radius of the planet were incorrect.”

Worked example 12 marks

The average orbital speed v of an object in the Solar System is defined by the equation v = (2π r)/T. Define the terms r and T in this equation.

  1. 1

    r is the radius of the orbit (of the object) [1]

  2. 2

    T is the orbital period, the time taken to complete one orbit [1]

5. Using the equation: period in seconds

Rearrange to find the orbital period: T = (2π r)/v.

The Earth's period is approximately 365 days. Convert days to seconds when v is in m/s and r in metres: 365 × 24 × 60 × 60 s. Do not round until the end.

T = (2π r)/v

Examiner insight

“Weaker candidates often incorrectly chose option C as they did not convert 365 days into seconds.” “A few candidates rounded intermediate calculations during their proof.”

Worked example 14 marks

The average orbital speed of Saturn is 9600 m / s. Saturn is 1.4 × 10¹² m from the Sun. Show that one Saturn year is approximately equal to 30 Earth years.

  1. 1

    T = (2π r)/v [1]

  2. 2

    T_Saturn = (2π × 1.4 × 10¹²)/9600 = 9.16 × 10⁸ s [1]

  3. 3

    T_Earth = 365 × 24 × 60 × 60 = 3.15 × 10⁷ s [1]

  4. 4

    (9.16 × 10⁸)/(3.15 × 10⁷) = 29 Earth years, which is approximately 30 [1]

6. Orbit above the Earth's surface

For a satellite, r is measured from the centre of the Earth. It is the height above the surface plus the radius of the Earth.

Use the period given. A satellite that completes one orbit every 24 hours has T = 24 × 3600 s.

Examiner insight

“Some candidates made errors such as not including the radius of the Earth or using a time equivalent to one hour instead of 24 hours.”

Worked example 11 mark

A satellite is orbiting above the Earth's equator. It completes one orbit of the Earth every 24 hours. The satellite is 36 000 km above the surface of the Earth and the radius of the Earth is 6400 km. What is the orbital speed of the satellite?

A 2.6 m / s · B 3.1 km / s · C 74 km / s · D 470 km / s

  1. 1

    B: r = 36 000 + 6400 = 42 400 km, T = 24 × 3600 = 86 400 s, v = (2π × 42 400)/(86 400) = 3.1 km/s [1]

Recap

  • The Earth rotates on its tilted axis once in about 24 hours: day and night.
  • The Earth orbits the Sun once in about 365 days: the seasons.
  • The Moon orbits the Earth once in about one month: the cycle of phases.
  • Rotation is spin on the axis. Orbit is travel round the Sun.
  • Extended: v = (2π r)/T, with r the average radius of the orbit and T the orbital period.
  • Extended: for a satellite, r = height + radius of the Earth. Use T in seconds.

7. What is in the Solar System

The Solar System contains one star, the Sun. Eight named planets orbit it. In order from the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Minor planets also orbit the Sun. They include dwarf planets such as Pluto, and asteroids in the asteroid belt.

Moons orbit the planets. Smaller bodies include comets and natural satellites.

Examiner insight

“Some candidates referred to dwarfs and omitted the word planets.”

Worked example 11 mark

Which statement about the Solar System is correct?

A All the planets are rocky · B Only the Earth has a moon · C Pluto is a dwarf planet · D There are many stars in the Solar System

  1. 1

    C: Pluto is a dwarf planet. [1]

8. Rocky planets and gas giants

The four planets nearest the Sun are rocky and small. The four planets furthest from the Sun are gaseous and large.

The accretion model explains this difference. It depends on gravity. Interstellar clouds of gas and dust contain many elements. The material in the cloud rotates and forms an accretion disc.

Examiner insight

“Very few candidates gave a detailed description of planetary formation using the accretion model from the syllabus. A significant number described the life cycle of a star.”

9. Gravitational field strength

The strength of the gravitational field at the surface of a planet depends on the mass of the planet.

Around a planet, the field decreases as the distance from the planet increases.

Examiner insight

“Although many stronger candidates answered this question correctly, a large number of candidates had the misconception that gravitational field strength depends on atmospheric pressure and therefore chose option A.” “A common incorrect answer was the distance of each planet from the Sun.”

10. Light travel time

Light, radio signals and microwaves all travel at the same high speed. To find the time for light to travel between objects in the Solar System, use distance = speed × time.

A signal that goes to an object and back has travelled the distance twice.

distance = speed × time

t = d/v

Examiner insight

“Only a few noticed that the distance calculated was twice the Earth–Moon distance and needed to be halved.” “The most common error was to use an incorrect rearrangement of the equation or to add powers of ten instead of subtracting for division.”

Worked example 13 marks

A spacecraft on the Moon sends a radio signal to the Earth. The radio signal takes 1.28 s to travel from the Moon to the Earth. The speed of the radio signal is 3.0 × 10⁸ m / s. Calculate the distance from the Moon to the Earth.

  1. 1

    distance = speed × time [1]

  2. 2

    = 3.0 × 10⁸ × 1.28 [1]

  3. 3

    = 3.8 × 10⁸ m [1]

11. Why the planets orbit the Sun

The Sun contains most of the mass of the Solar System. This explains why the planets orbit the Sun.

The force that keeps an object in orbit around the Sun is the gravitational attraction of the Sun.

Examiner insight

“Common errors included gravitational potential energy or just ‘attraction’.” “There were a few who thought it was electromagnetic or just an orbital force.”

Worked example 11 mark

State the name of the force that keeps the planets in orbit around the Sun.

  1. 1

    gravitational attraction [1]

12. Elliptical orbits

Planets, minor planets and comets have elliptical orbits. The Sun is not at the centre of the ellipse.

The exception is when the orbit is approximately circular.

Examiner insight

“Although most candidates knew the shape of a comet’s orbit as elliptical, many had the misconception that the Sun is positioned at the centre of a comet’s orbit.” “Approximately circular was an acceptable answer but circular alone was insufficient and two responses with one incorrect e.g., elliptical/circular were not awarded credit.”

Worked example 12 marks

The average orbital speed of Saturn is 9600 m / s. Explain why the orbital speed is an average value.

  1. 1

    the orbit is not exactly circular [1]

  2. 2

    the distance from the Sun varies [1]

13. Planetary data and distance from the Sun

Planetary data gives orbital distance, orbital duration, density, surface temperature and the gravitational field strength at the planet's surface. Read the right planet and the right column.

The Sun's gravitational field decreases as the distance from the Sun increases. The orbital speeds of the planets decrease too.

Examiner insight

“Stronger candidates explained that this faster speed is due to the stronger gravitational field closer to the Sun.” “When discussing the gravitational field strength, stronger candidates were able to relate Jupiter’s large mass to its large value of gravitational field strength but weaker candidates just associated Jupiter’s large size with its large gravitational field strength.”

14. Speed in an elliptical orbit

An object in an elliptical orbit travels faster when closer to the Sun and slower when further away.

The explanation uses conservation of energy. Moving away from the Sun, kinetic energy decreases and gravitational potential energy increases. Moving closer, the transfer is the other way.

Examiner insight

“Many candidates correctly stated the energy transfer without any reference to conservation of energy.” “A few candidates noted that total energy is conserved during the transfers between GPE and KE.”

Recap

  • One star (the Sun), eight planets, minor planets (dwarf planets, asteroids), moons, comets.
  • Nearest four planets: rocky and small. Furthest four: gaseous and large.
  • Surface field strength depends on the mass of the planet. Field decreases as distance increases.
  • Light travel: distance = speed × time. A return trip is twice the distance.
  • The Sun has most of the mass. Gravitational attraction of the Sun keeps planets in orbit.
  • Extended: orbits are elliptical, with the Sun not at the centre.
  • Extended: the Sun's field and the planets' orbital speeds decrease as distance from the Sun increases.
  • Extended: closer to the Sun means faster, by conservation of energy.

End-of-chapter exercise

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

  1. State the approximate time taken for the Earth to rotate once on its axis.1 mark
  2. A student says that day and night happen because the Sun travels round the Earth once every 24 hours. Give the correct explanation.2 marks
  3. A student says that it is winter because the Earth is further from the Sun. Give the correct reason for the seasons.2 marks
  4. Explain why the Moon's phases repeat in a regular cycle. State how long one cycle takes.2 marks
  5. The Earth orbits the Sun twice. How many times does it rotate on its axis in this time?3 marks
  6. A planet orbits a star at an average radius of 3.0 × 10¹¹ m. Its orbital period is 4.0 × 10⁷ s. Calculate its average orbital speed.3 marks
  7. A satellite orbits at an average speed of 7.5 km / s. Its orbital radius is 6800 km. Calculate its orbital period in minutes.3 marks
  8. A satellite is 20 000 km above the Earth's surface and completes one orbit every 12 hours. The radius of the Earth is 6400 km. Calculate its average orbital speed in km / s.4 marks
  9. A spacecraft orbits the Moon once every 2.0 hours at an average orbital speed of 1.6 km / s. The radius of the Moon is 1740 km. Calculate the height of the spacecraft above the Moon's surface.4 marks
  10. Venus orbits the Sun at an average radius of 1.1 × 10¹¹ m with an orbital period of 225 days. The Earth orbits the Sun at an average radius of 1.5 × 10¹¹ m. Calculate the average orbital speed of each planet in m / s. Then calculate how many times faster Venus moves than the Earth.5 marks
  11. State two types of object, other than planets and moons, that orbit the Sun.2 marks
  12. Which planet is fifth from the Sun: Mars, Jupiter, Saturn or Uranus?1 mark
  13. Compare the four planets nearest the Sun with the four planets furthest from the Sun.2 marks
  14. State why the planets orbit the Sun. Name the force that keeps them in orbit.2 marks
  15. Describe how the accretion model explains the formation of the planets.3 marks
  16. Planet X has a greater mass than planet Y. Planet Y is nearer to the Sun than planet X. State which planet has the stronger gravitational field at its surface. Give a reason.2 marks
  17. A probe is 6.0 × 10¹⁰ m from the Earth. It sends a radio signal to the Earth. The speed of the signal is 3.0 × 10⁸ m / s. Calculate the time taken for the signal to reach the Earth.3 marks
  18. A laser pulse is sent from the Earth to a reflector on the Moon. It returns after 2.56 s. The speed of light is 3.0 × 10⁸ m / s. Calculate the distance from the Earth to the Moon.3 marks
  19. The table gives data for four planets, A to D, that orbit the same star. The planets formed in the same way as the planets of the Solar System. (a) State which two planets are gaseous. Use data from the table to give a reason. (b) A student says planet D has a weaker surface field than planet C because it is further from the star. State what is wrong with this. Give the correct reason. (c) At one time the star, planet B and planet C lie on a straight line, with both planets on the same side of the star. A radio signal is sent from planet B to planet C. The speed of the signal is 3.0 × 10⁸ m / s. Calculate the time taken for the signal to reach planet C. (d) State which planet has the greatest orbital speed. Explain your answer. | Planet | orbital distance / 10⁶ km | density / g/cm³ | surface temperature / °C | field strength / N/kg | | --- | --- | --- | --- | --- | | A | 58 | 5.4 | 430 | 3.7 | | B | 150 | 5.5 | 15 | 9.8 | | C | 780 | 1.3 | −110 | 25 | | D | 4500 | 1.6 | −200 | 11 |9 marks
  20. An asteroid has an elliptical orbit around the Sun. It is closest to the Sun at point P and furthest from the Sun at point Q. At P its kinetic energy is 9.0 × 10¹² J. From P to Q, its gravitational potential energy increases by 6.5 × 10¹² J. (a) Describe how the speed of the asteroid changes during one complete orbit, starting at P. (b) Calculate the kinetic energy of the asteroid at Q. State the principle you use. (c) Point R is between P and Q. At R the gravitational potential energy is 2.5 × 10¹² J less than at Q. Calculate the kinetic energy at R. State whether the asteroid is faster at R or at Q. (d) A student draws the orbit with the Sun at its centre. Use P and Q to state why this is wrong.8 marks

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