The scale and members of the Solar System
The largest body of the Solar System is the Sun: its diameter is ≈ 109 times and its mass ≈ 333,000 times the Earth’s. Around it 8 planets – Mercury, Venus, Earth, Mars (terrestrial) and Jupiter, Saturn, Uranus, Neptune (giants) – revolve in nearly one plane; the asteroid belt lies between Mars and Jupiter and the Kuiper belt beyond Neptune. In 2006 the International Astronomical Union (IAU) defined a planet as a body that orbits the Sun, is nearly round because of its own gravity and has cleared the neighbourhood around its orbit; Pluto fails the third condition and is a dwarf planet. Distances are measured in astronomical units (AU): the mean Earth–Sun distance, ≈ 149.6 million km. Mean distances of the planets from the Sun: Mercury 0.39 AU, Venus 0.72, Earth 1, Mars 1.52, Jupiter 5.2, Saturn 9.6, Uranus 19.2, Neptune 30.1 AU. The speed of light is ≈ 300,000 km/s, so sunlight reaches the Earth in ≈ 500 s (8 min 20 s) and Neptune in about 4 hours. The heliopause, where the Sun’s wind stops dominating, lies at ≈ 120 AU; Voyager 1 crossed it in 2012 and Voyager 2 in 2018. Voyager 1 is expected to reach about one light-day (≈ 173 AU) from the Sun around the end of 2026. Scale: if the Sun were a ball 14 cm across, the Earth would be a 1.3 mm speck 15 m away, and Neptune would be ≈ 450 m away.
A scale model in the school yard or hall: take 1 AU = 1 m. Mark the Sun with a pea-sized (≈ 9 mm) paper disc and place the Earth 1 m from it, Jupiter ≈ 5 m and Neptune ≈ 30 m away. Measure the distances with a tape measure (paper markers only, nothing sharp).