The shape, size and internal structure of the Earth: the oblate spheroid, the mean diameter of 13000 km, and the crust, mantle, outer core and inner core.

What this dot point is asking

Edexcel statements 1.1 to 1.3 want you to know the shape of the Earth (an oblate spheroid), to use information about its mean diameter of 13000 km in calculations, and to describe the four major internal divisions of the Earth (crust, mantle, outer core and inner core) and their features.

The shape of the Earth

The difference is small (the equatorial diameter is only about 43 km larger than the polar diameter), so the Earth looks spherical in photographs, but "oblate spheroid" is the precise answer the mark scheme wants. Calling it a perfect sphere is the classic lost mark. The same flattening happens to other rotating bodies, and faster-spinning planets such as Jupiter and Saturn are visibly more oblate.

The size of the Earth

The mean diameter is a single average value used because the Earth is not perfectly round. Questions often ask you to combine it with $\pi$ to get the circumference, or to take a ratio with the Moon's mean diameter of 3500 km (so the Earth is about 3.7 times wider than the Moon) or the Sun's mean diameter of $1.4 \times 10^6\,\text{km}$. Working in standard form, $13000\,\text{km} = 1.3 \times 10^4\,\text{km}$.

The internal structure of the Earth

The layers differ in state and composition: rocky crust and mantle outside, metallic iron and nickel cores inside. The boundary between solid inner core and liquid outer core is set by pressure, not just temperature, which is why the deeper inner core is solid even though it is hotter. The liquid outer core, being a moving conductor, is the source of the Earth's magnetic field, which links forward to the magnetosphere in solar astronomy.

How Edexcel examines this

This is naked-eye Paper 1 content and a reliable source of recall and short-calculation marks. The shape question rewards naming the oblate spheroid and linking the equatorial bulge to rotation; do not write "sphere" alone. The diameter is given on the data sheet, so calculations test whether you can use it: expect circumference ($C = \pi d$), radius ($d/2$), or a ratio comparing the Earth with the Moon or Sun, sometimes in standard form. The structure question rewards the four divisions in the correct order with their states, especially the liquid outer core and solid inner core. A frequent synoptic link is to the magnetosphere (the moving liquid outer core generates the magnetic field) in Topic 10, so be ready to connect the inner structure to that. Keep the answers precise and ordered, and watch the units (kilometres throughout).

Try this

Q1. State the precise name for the shape of the Earth. [1 mark]

• Cue. An oblate spheroid (flattened at the poles, bulging at the equator).

Q2. Name the four major internal divisions of the Earth in order from the surface. [2 marks]

• Cue. Crust, mantle, outer core, inner core.