The solar system and beyond
Physics · Key Stage 3 · The School
Order and scale
Mercury, Venus, Earth, Mars, then the asteroid belt, then Jupiter, Saturn, Uranus, Neptune. The four inner planets are small and rocky; the four outer are giants. Scale is the part diagrams get wrong: if Earth were a peppercorn, the Sun would be a beach ball 26 metres away, and Neptune would be nearly a kilometre off.
Mass and weight are different
MASS is how much matter there is, measured in kilograms, and it does not change. WEIGHT is the force of gravity on that mass, measured in NEWTONS, and it does change: W = m × g. On the Moon g is about 1.6 N/kg instead of 9.8, so an astronaut weighs about a sixth as much while having exactly the same mass.
Reading starlight
We have never visited a star, yet we know what they are made of. Each element absorbs specific wavelengths, leaving dark lines in the star's spectrum like a barcode. Helium was found in the Sun's spectrum before it was found on Earth — which is why it is named after Helios.
An astronaut has a mass of 70 kg. On Earth, g = 9.8 N/kg, so weight = 70 × 9.8 = 686 N. On the Moon, g = 1.6 N/kg, so weight = 70 × 1.6 = 112 N — about a sixth. In orbit they float and appear weightless. Their MASS is 70 kg in all three places, because they contain exactly the same matter. Only the force of gravity on it changed.