The Universe is everything that exists in a material form. How big we say it is depends on how much we can actually 'see'.

Our Universe is made up of clusters of galaxies. The galaxies are not evenly spread out. Space is 'lumpy'. We are aware of about one hundred billion of galaxies. Each of the galaxies contains hundreds of billions of stars.

The stars swirl around the centre of the galaxy and there is thought to be a black hole at the centre of each galaxy.

Our galaxy is called the Milky Way.

A star is the only thing in the universe that gives out its own light. We see other 'heavenly bodies' by reflected light, but stars give out their own light. The star does not burn (although poets often say they do!). A nuclear reaction called nuclear fusion within that star produces the energy that the star gives out. You will learn about that in KS4.

Our star is called Sol (or the Sun)

Stars are often orbited by planets. They do not emit their own light - they are seen by light from the star they orbit. Its light reflects off the surface of the planet.

Planets spin on their own axis - the time taken to spin once in its axis is called the planet's day.

Planets 'go round' the star - they orbit it. The time taken for a planet to complete one orbit of the star is called its year. Planets take longer to complete an orbital path the greater the distance they are from the star. In other words the further out the planet is the longer its year will be.

The tilt of a planet's axis responsible for seasons on a planet.


Planets orbit in an 'oval shaped' path called an ellipse. Our planet (Earth) has an orbit that is almost circular - but note quite. It is actually closer to the Sun in January and further away in July. Look at the animation below - notice the planet moves faster when close to the Sun and slower when further away - notice also that the outer planet has a longer orbital time period than the inner planet.

The star and all of the bodies orbiting it are called that star's system. Our star is Sol so our system is the Solar system. In Star Wars (TM) Luke has to go to the Degobah system to find the planet that Yoda is on,

You are expected to recall the names of the planets in our solar system in order - a mnemonic can help you do that. In 2006 Pluto was 'demoted' from being a planet to being a 'dwarf planet'. When it was discovered in 1930 it was called a planet. It had a moon and an atmosphere (like many of the other planets) and although it was small and had an very elliptical orbit, people called it a planet... but current classification calls it a dwarf planet.

There are other bodies orbiting our Sun. They have regular (predictable orbit) - see comets and asteroids.

The atmosphere of a planet or natural satellite (moon) is made up of gases - held onto the planet's surface by the force of gravity. Very small planets rarely have an atmosphere. We call our atmosphere 'the air'. It is made up of a mixture of gases.

Other planets have different gases in the mixture.

Whether a gas is in the atmosphere depends on the temperature of the planet's surface. Gases turn into liquids if it is too cold. You have to look at the boiling point of the gas to see if it will be a liquid or a gas on the planet's surface.

Some gases are acidic - e.g. sulphur dioxide - and they cause acid rain; other gases e.g. carbon dioxide help the atmosphere hold onto the energy it gets from the Sun. Those are called greenhouse gases.

Moons or natural satellites orbit planets. We only see a portion of the moon at certain times because we only see the reflected light from the moon's surface, and how much of it we see depends on our position relative to the illuminated 'bit'.

You need to know the terms:

New moon - no light can be seen reflected from the moon

Full moon - a full circle of reflected light can be seen

First quarter and Last quarter - half circle of reflected light can be seen.

Gibbous and Crescent

See 'phases of the moon' for fuller details.

As the satellite orbits the planet you sometimes get an eclipse. This happens when the one heavenly body blocks the light from the other one.

In a lunar eclipse the Earth blocks the sunlight falling on the moon (Earth in the middle).

In a solar eclipse the moon blocks the view of the Sun (Moon in the middle)

Eclipses are regular - we can predict them because there are fixed time intervals between our seeing them. You will therefore find them predicted in calendars and diaries.

The above link takes you to the Y7 revision Flashcard Topics.

These may help you revise for the test.