Earth in Space

Look up at the Sun (never directly!) and you are looking at a star — a giant ball of hot, glowing gas. It is the centre of our solar system.

The solar system is made up of:

• The Sun — a star at the centre. It gives out light and heat.
• Planets — eight large worlds that travel around the Sun.
• Moons — smaller objects that travel around planets.
• Smaller bits and pieces, like asteroids and comets.

Everything in the solar system moves around the Sun in a path called an orbit. The Sun's huge gravity is what holds everything in orbit and stops the planets from drifting away into space.

The Sun is by far the biggest object in the solar system. It is so large that more than a million Earths could fit inside it. It is also our most important energy source — without the Sun's light and heat, there would be no life on Earth.

Eight planets orbit the Sun. Knowing their order — from nearest to farthest — is a useful place to start.

The order from the Sun is: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

There are two types of planet:

• The four nearest the Sun — Mercury, Venus, Earth, Mars — are smaller and rocky.
• The four farthest out — Jupiter, Saturn, Uranus, Neptune — are huge and made mostly of gas.

A planet closer to the Sun, like Mercury, has a much shorter orbit, so its year is short. A planet far out, like Neptune, takes a very long time to go round once.

The Sun seems to rise, cross the sky and set. But the Sun is not really moving across our sky — the Earth is spinning.

The Earth turns around an imaginary line through its centre called the axis. One full spin takes about 24 hours — one day.

At any moment, only one side of the Earth faces the Sun:

• The side facing the Sun is lit up — it is daytime there.
• The side facing away is in shadow — it is night-time there.

As the Earth keeps spinning, your part of the world turns into the light (sunrise), then back into the dark (sunset), over and over. So day and night are caused by the Earth's spin, not by the Sun moving.

As well as spinning, the Earth travels around the Sun in a giant orbit. One full orbit takes about 365 days — one year.

Why do we get seasons? It is because the Earth's axis is tilted. It does not point straight up — it leans over slightly. This tilt stays pointing the same way all year as the Earth orbits.

Because of the tilt:

• For part of the year, your half of the Earth is tilted towards the Sun. The Sun is higher in the sky, days are longer, and it is summer.
• For another part of the year, your half is tilted away from the Sun. The Sun is lower, days are shorter, and it is winter.

When the northern half of the Earth has summer, the southern half has winter — and the other way around. That is why countries on opposite sides of the Earth have opposite seasons at the same time.

So: the spin gives us day and night, and the tilted orbit gives us the year and the seasons.

The Moon is the Earth's natural satellite — it orbits the Earth, taking about a month to go round once.

The Moon does not make its own light. It reflects light from the Sun. The Sun always lights up one half of the Moon, but from Earth we see different amounts of that lit half. These changing shapes are called phases.

Some of the main phases are:

• New moon — the lit side faces away from us, so the Moon looks dark.
• Crescent moon — only a thin sliver of the lit side is visible.
• Half moon — we see half of the lit side.
• Full moon — the whole lit side faces us, so the Moon looks like a bright circle.

The phases repeat in a cycle that takes about a month — which is where the idea of a "month" comes from.

Knowing how the Earth moves in space helps with many later topics:

• Gravity and forces — gravity is the force that holds planets and moons in their orbits.
• Light — understanding that the Moon reflects light, and how shadows work, builds on the idea of light sources.
• Space exploration — knowing the order of the planets helps you make sense of space missions and discoveries.
• Climate and weather — the seasons, caused by the Earth's tilt, shape weather patterns around the world.

The next time you watch a sunrise or notice the Moon's shape, you will understand the huge movements in space behind it. Take your time with this topic — it changes how you see the sky.