Planet Earth: Structure and Reference Points

The planet you observe from 🌍

Every observation you make in astronomy is made from the surface of the Earth, so it pays to know the Earth itself well. It is not a perfect sphere. Because it spins, it bulges slightly at the Equator and is flattened at the poles. That shape has a name: an **oblate spheroid**.

How big is the Earth?

An interactive activity.

What shape?

What does it mean to call the Earth an "oblate spheroid"?

• A sphere slightly flattened at the poles and bulging at the Equator
• A perfect sphere, the same all the way round
• An egg shape, pointed at one end
• A flat disc

Inside the Earth 🧅

Cut the Earth open and you find four layers, from the surface inwards: - **Crust**: the thin, solid, rocky surface. - **Mantle**: the thick layer of hot, slowly flowing rock. - **Outer core**: a layer of liquid iron and nickel. - **Inner core**: a ball of solid iron and nickel at the centre, the hottest part.

Find the inner core

An interactive activity.

From surface to centre

An interactive activity.

Solid or liquid?

Which layer is liquid?

• The outer core
• The inner core
• The crust
• The mantle

Mapping the Earth 🧭

To say exactly where a place is, we use two sets of lines: - **Latitude** lines run east-west (parallel to the Equator) and measure how far north or south you are. - **Longitude** lines run north-south (pole to pole) and measure how far east or west you are. The **Equator** is latitude 0 degrees; the **Prime Meridian** is longitude 0 degrees.

Match each reference point to its position

Which line is zero?

Which line is defined as 0 degrees longitude, the starting line for measuring east and west?

• The Prime Meridian
• The Equator
• The Tropic of Cancer
• The Arctic Circle

The air in the way 🌫️

Sunlight and starlight must pass through the atmosphere before it reaches your eye, and the air changes what you see: - **Sky colour**: air scatters blue light most, so the daytime sky looks blue. - **Skyglow**: wasted artificial light scatters in the air and hides faint stars. This is **light pollution**. - **Twinkling**: moving pockets of air bend starlight, so stars seem to shimmer. Astronomers call this **seeing**.

Match each effect to its cause

Good observing site

You want the clearest possible view of faint stars. Select the TWO conditions that would help.

• Little light pollution (dark skies)
• Steady air (good seeing)
• A bright city centre
• Very turbulent air

Planet Earth summary

The Earth is an oblate _____, about 13,000 km across. Its layers from the surface in are crust, mantle, outer core and inner _____. The Equator is latitude 0 degrees and the Prime _____ is longitude 0 degrees. Wasted artificial light causes _____, which hides faint stars.