Key Stage 3 science · Physics – Space physics

KS3 gravity, stars, galaxies and the seasons: a parent guide

Space physics at Key Stage 3 combines four ideas: gravity and the difference between mass and weight, the Sun as one star among billions in a galaxy, the light year as a way to measure huge distances, and the reasons we have seasons and changing day length. Pupils start to use a simple equation, weight = mass × gravitational field strength, and to explain things they have seen all their lives, such as why summer days are long.

Gravity, stars, galaxies and seasons, explained

Gravity is a force of attraction between any two objects that have mass. We only notice it when at least one object is enormous, like a planet. Pupils learn the difference between mass and weight, which everyday language blurs. Mass is the amount of matter in an object, measured in kilograms, and it is the same everywhere. Weight is the pull of gravity on that mass, a force measured in newtons. The two are linked by weight = mass × gravitational field strength, or W = m × g, where g is measured in newtons per kilogram. On Earth the curriculum uses g = 10 N/kg, although the more exact figure is about 9.8. Other places have different values: on the Moon g is about 1.6 N/kg, on Jupiter about 25 N/kg and at the surface of the Sun about 270 N/kg, so the same person would weigh much less on the Moon and far more on Jupiter or the Sun.

Pupils are also asked to describe gravity between bodies in space, in words only and with no calculation. The bigger the masses and the closer they are, the stronger the pull. Gravity between the Earth and the Moon keeps the Moon in orbit around the Earth. Gravity between the Sun and the Earth keeps the Earth, and the other planets, in orbit around the Sun, which is far more massive than any of them. Your weight is the pull of gravity from the planet, moon or star you are on. The Sun itself is a star: an enormous ball of hot gas that gives out light and heat. It looks different from the other stars only because it is so much closer to us.

The other stars we see at night are other suns, many of them like ours, but so far away that they look like points of light. Our Sun is one of billions of stars in our galaxy, called the Milky Way, which is held together by gravity. There are billions of other galaxies. Distances in space are so large that kilometres become awkward, so astronomers use the light year, the distance that light travels in one year, which is about 9.5 million million kilometres. Despite the name, it is a unit of distance, not time. The nearest star after the Sun is about 4.2 light years away, our galaxy is roughly 100,000 light years across, and the Andromeda galaxy is around 2.5 million light years away. Looking far away is also looking back in time, because the light we see left its source long ago.

The seasons are caused by the tilt of the Earth’s axis, not by its distance from the Sun. The Earth’s axis is tilted at about 23.5 degrees and always points the same way in space as the Earth makes its year-long orbit. In June, the northern hemisphere is tilted towards the Sun. The Sun then climbs higher in the sky, its light hits the ground more directly and is more concentrated, and the days are longer, so the weather is warmer. Six months later the northern hemisphere is tilted away, the Sun stays low, and the days are short and cool. The seasons are reversed in the southern hemisphere, so Australia has summer at Christmas. In fact the Earth is slightly closer to the Sun in early January, during the UK’s winter.

Day length follows from the tilt. In the south of England the longest day, around 21 June, is about 16 and a half hours of daylight, and the shortest, around 21 December, is about 8 hours. Further north the differences are bigger. At the equator, daylight stays at about 12 hours all year, and near the poles the Sun does not set at all for part of the summer and does not rise for part of the winter. Pupils are asked to explain these patterns with labelled diagrams of the tilted Earth and the Sun, and to predict how the pattern would look in the southern hemisphere.

Words your child will hear

Mass: The amount of matter in an object, measured in kilograms. It does not change from place to place.
Weight: The force of gravity acting on an object, measured in newtons.
Gravitational field strength: How strong gravity is in a place, in newtons per kilogram (N/kg). It is written as g.
Galaxy: A huge collection of stars, gas and dust held together by gravity. Ours is the Milky Way.
Light year: The distance light travels in one year, about 9.5 million million kilometres.
Axis: The imaginary line through the North and South Poles that the Earth spins around.
Hemisphere: One half of the Earth, north or south of the equator.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about gravity force, weight = mass x gravitational field strength (g), on Earth g = 10 N/kg, different on other planets and stars; gravity forces between Earth and Moon, and between Earth and Sun (qualitative only)
Pupils should be taught about our Sun as a star, other stars in our galaxy, other galaxies
Pupils should be taught about the seasons and the Earth's tilt, day length at different times of year, in different hemispheres
Pupils should be taught about the light year as a unit of astronomical distance

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

An astronaut has a mass of 50 kg. Using g = 10 N/kg on Earth and 1.6 N/kg on the Moon, find the astronaut’s weight in both places, and say what happens to the mass.

Pick the right equation. Weight is a force, found from mass and gravitational field strength.
```
weight = mass × g
W = m × g
```
On Earth, put in the numbers. The mass is 50 kg and g is 10 N/kg.
```
W = 50 × 10
W = 500 N
```
On the Moon, the mass is still 50 kg but g is 1.6 N/kg.
```
W = 50 × 1.6
W = 80 N
```
Compare the two. 80 ÷ 500 = 0.16, which is a bit under one sixth. The astronaut weighs about one sixth as much on the Moon, which is why astronauts can bounce along.
Write down what stays the same. Mass is the amount of matter in the astronaut, so it is 50 kg on Earth, on the Moon and in orbit. Only the weight changes.

The astronaut weighs 500 N on Earth and 80 N on the Moon. The mass is 50 kg in both places.

Common mistakes and how to help

Using mass and weight as if they mean the same thing

Bathroom scales are marked in kilograms and people say “I weigh 50 kilos”. In everyday language weight means mass.

What helps: Make the two words tell different stories. Mass is how much stuff there is (kg), weight is the pull on it (N). Ask what would change if you took the object to the Moon: the weight would, but not the mass.

Thinking there is no gravity in space

Pictures of astronauts floating make it look as if gravity has switched off.

What helps: Gravity is still there. Space stations orbit only a few hundred kilometres above the surface, where gravity is still around nine-tenths as strong as on the ground. The astronauts float because they and the station are falling around the Earth together.

Believing summer happens when the Earth is closer to the Sun

It seems logical that the nearer we are, the warmer it will be, and orbit diagrams are often drawn with stretched ovals that exaggerate the distance changes.

What helps: Point out that when it is summer in the UK it is winter in Australia, on the same planet at the same distance. Use a lamp and a tilted ball to show that the tilt changes how directly the light strikes the ground.

Thinking a light year is a measure of time

The word “year” makes it sound like a length of time.

What helps: Compare it with a phrase like “an hour’s drive”, which tells you a distance by saying how long it takes. A light year is how far light goes in a year, which is a very long way.

How to help at home

Lamp and ball model of the seasons. In a dark room, use a torch or lamp as the Sun and a ball or orange with a pencil pushed through it as the Earth. Tilt the pencil and keep it pointing the same way as you walk the ball round the lamp. Notice when the northern half is lit more directly.
Measure a shadow. Stand a stick upright in the garden or on a patio and mark the end of its shadow around the middle of the day each week, at about 12 noon in winter and 1 pm once the clocks have gone forward, when the Sun is close to its highest. Between December and June the shadow gets shorter as the midday Sun climbs higher. Never look straight at the Sun. Keep this to a safe outdoor space, away from roads.
Chart daylight hours. Using a weather forecast or a newspaper, write down the sunrise and sunset times on the same date each month and work out the length of the day. Ask your child to describe how the day length changes through the year, and to link it to the tilt of the Earth.
Weight versus mass in the kitchen. Weigh a bag of sugar on kitchen scales that read 1 kg. Ask your child to say what its mass is (1 kg) and to work out its weight on Earth using g = 10 N/kg (10 N), and on the Moon (1.6 N).
Look up on a clear night. With an adult, go outside on a clear evening and look for the brightest stars and the band of the Milky Way, if the sky is dark enough. Ask what the pinpricks of light are, and why some look brighter.

Leads on to

Key Stage 4: The solar system and orbits

Everything in Key Stage 3 science

Updated 20 October 2026.