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Topic 6 · 6.1

The Earth and the Solar System

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Use the Earth’s tilted, spinning axis and its orbit to explain day and night, the Sun’s apparent daily motion and the seasons; use the Moon’s orbit to explain its phases.
  • Describe the Solar System and the order of the planets; explain the rocky/gaseous difference with the accretion model.
  • Know how gravitational field strength depends on planet mass and distance; calculate light travel times; know the Sun holds most of the mass.
  • Use \( v = \dfrac{2\pi r}{T} \); describe elliptical orbits; analyse planetary data; explain orbital speed changes EXTENDED.

📚The physics

The Earth

  • The Earth rotates on its tilted axis once in about 24 hours: this gives day and night, and makes the Sun appear to rise in the east, move across the sky and set in the west.
  • The Earth orbits the Sun once in about 365 days. Because the axis is tilted, each hemisphere is tilted towards the Sun for part of the year (summer: longer days, Sun higher in the sky) and away for the other part (winter): the seasons.
  • The Moon orbits the Earth in about one month, giving the cycle of the Moon’s phases.
The Earth at two opposite points of its orbit round the Sun, with its axis tilted the same way in space. On the left, in June, the northern hemisphere is tilted towards the Sun: northern summer. On the right, in December, it is tilted away: northern winter.
The tilt stays pointing the same way in space all year.
The Moon at eight points of its orbit round the Earth, each with its sunward half lit. Between the Earth and the Sun it is a new moon; a quarter of the way round, first quarter; opposite the Sun, full moon; three quarters round, last quarter. The appearance from Earth is shown: dark, right half lit, fully lit, left half lit.
The phase depends on how much of the lit half faces the Earth.

The Solar System

The Sun and the eight planets in order, not to scale: Mercury, Venus, Earth and Mars, small and rocky; the asteroid belt; Jupiter, Saturn, Uranus and Neptune, large and gaseous; with the dwarf planet Pluto and comets beyond.
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

The Solar System contains one star (the Sun), the eight planets, minor planets (dwarf planets such as Pluto, and asteroids in the asteroid belt), moons orbiting planets, and smaller bodies such as comets. The four inner planets are small and rocky; the four outer ones are large and gaseous. The accretion model explains this: the Solar System formed from a rotating interstellar cloud of gas and dust containing many elements; gravity pulled it together into an accretion disc around the young Sun; near the hot Sun only rocky materials could condense, while further out gases and ices collected into giant planets.

  • The gravitational field strength at a planet’s surface depends on the planet’s mass; around it, the field gets weaker with distance.
  • The Sun contains most of the mass of the Solar System, so its gravitational attraction keeps the planets in orbit.
  • Light (speed 3.0 × 108 m/s) takes about 8.3 minutes to reach us from the Sun: time = distance ÷ speed.

Orbits EXTENDED

\[ v = \frac{2\pi r}{T} \]

EXTENDED r is the average orbital radius and T the orbital period. Planets, minor planets and comets have elliptical orbits with the Sun not at the centre (except for nearly circular orbits). The Sun’s field gets weaker further out, so the orbital speeds of the planets decrease with distance from the Sun.

An elliptical orbit with the Sun at one focus near one end. The orbiting body moves fastest at its closest point and slowest at its furthest point.
EXTENDED Conservation of energy: gravitational potential energy and kinetic energy swap as the distance changes.
A graph of average orbital speed against average orbital radius for Mercury, Venus, Earth, Mars, Jupiter and Saturn: the speed falls from about 47 kilometres per second for Mercury to about 10 for Saturn, along a smooth curve.
EXTENDED Planetary data: the further out, the slower (approximate values).

✏️Worked example

EXTENDED The planet Saturn orbits the Sun at an average radius of 1.43 × 109 km with an average orbital speed of 3.49 × 104 km/h. (Modelled on 0625/42 June 2026 Q11(b).) (a) Show that its orbital period is about 29 years. [3] (b) Light from the Sun takes 8.3 minutes to reach the Earth. Calculate how long it takes to reach Saturn. [2]

(a) \( T = \dfrac{2\pi r}{v} = \dfrac{2\pi \times 1.43 \times 10^{9}}{3.49 \times 10^{4}} = 2.57 \times 10^{5} \) hours; \( \dfrac{2.57 \times 10^{5}}{24 \times 365} = \) 29.4 years.

(b) t = d/v = 1.43 × 1012 m / 3.0 × 108 m/s = 4770 s = 79 minutes (1.3 hours).

Check it. Saturn is about 9.5 times further than the Earth, so light should take about 9.5 × 8.3 ≈ 79 minutes. It agrees.
Mixing units. Here r is in km and v in km/h, so T comes out in hours; convert to years at the end (8760 hours per year).

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Theory.) Match each event to its approximate time: the Earth orbits the Sun; the Earth rotates once; the Moon orbits the Earth. Times: 24 hours, 1 month, 365 days. [2]
Earth orbits the Sun: 365 days. Earth rotates: 24 hours. Moon orbits the Earth: 1 month.
2. (Theory.) A rover on Mars sends a radio signal to the Earth when Mars is 7.8 × 1010 m away. Radio waves travel at 3.0 × 108 m/s. Calculate the time the signal takes. [2] (Modelled on 0625/32 June 2026 Q10(c).)
t = d/v = 7.8 × 1010 / 3.0 × 108 = 260 s (about 4.3 minutes).
3. (Theory.) Name the force that keeps the Moon in orbit around the Earth. [1]
Gravity (the gravitational attraction of the Earth).
4. (Theory.) Explain why the four planets nearest the Sun are small and rocky. [3]
In the accretion disc, it was hot near the young Sun, so only rocky materials (with high melting points) could condense; gases were driven outwards. Only small amounts of rock were available, so the inner planets are small; further out, cooler conditions let gases and ices gather into large gaseous planets.
5. (Theory.) Explain why it is summer in the southern hemisphere in December. [2]
The Earth’s axis is tilted; in December the southern hemisphere is tilted towards the Sun, so it receives more direct sunlight for longer each day.
6. (Multiple choice.) EXTENDED Which expression gives the orbital period T of the Moon, orbital radius r and speed v? A: 2πrv. B: 2πr/v. C: 2πr2/v. D: v/2πr.
B.
7. (Theory.) EXTENDED A comet has a very elliptical orbit. Explain, using the conservation of energy, why it moves fastest when closest to the Sun. [3]
As it falls towards the Sun, its gravitational potential energy decreases; energy is conserved, so its kinetic energy increases and it speeds up. It is fastest at its closest point, where the potential energy is least.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • NASA Solar System Exploration — planetary data
  • PhET “Gravity and Orbits” — orbital speed and distance