Planet Earth: Structure and Reference Points

The planet you observe from

Every observation in this course is made from the surface of a spinning, layered ball wrapped in moving air, and each of those three facts gets in the way of seeing clearly. Start with the spin. Anything rotating throws its material outwards, and the Earth is no exception: it is widest around the Equator and flattened at the poles. The name for that shape is an oblate spheroid. The next two steps are about how much difference the spin actually makes.

How much of a bulge?

Measured through the Equator the Earth is 12 756 km across. Measured from pole to pole it is 12 714 km. How much wider is it across the Equator, in km?

Could you see it?

That bulge of 42 km is about 0.3% of the Earth's diameter. What follows from a figure that small?

• The flattening is real and easily measured, but far too slight to see. A photograph of the Earth from space looks perfectly round, and textbook diagrams showing a visibly squashed planet are exaggerating to make the point
• The Earth is noticeably egg-shaped in photographs taken from orbit
• It is so small that astronomers treat the Earth as a perfect sphere for every purpose
• It shows the Earth is slowly shrinking at the poles

Inside the Earth

Four layers, from the surface inwards. The states matter as much as the names:

Find the inner core

This is a cutaway of the Earth. Tap the inner core.

How do we know?

The deepest hole ever drilled reached about 12 km, which is not even through the crust. So how do we know the outer core is liquid?

• From earthquakes. One kind of seismic wave cannot travel through a liquid at all, and that kind never arrives on the far side of the Earth, leaving a shadow zone the size and shape of the outer core
• Deeper boreholes have reached it in a few places where the crust is thinnest
• It is an assumption based on how hot it must be at that depth
• Because the Earth has a magnetic field, and only a liquid can be magnetic

Two sets of lines

Any point on Earth needs two numbers. The two sets of lines are not mirror images of each other, and the differences are examinable:

Where are you?

You measure Polaris sitting 51 degrees above your northern horizon. What does that tell you, and how would the measurement change if you drove to Edinburgh?

• Your latitude is 51° N, which puts you around southern England. Driving north to Edinburgh would raise Polaris to about 56°, because the pole star climbs as your latitude increases
• Your longitude is 51° E, and driving to Edinburgh would barely change it
• Your latitude is 51° N, and Polaris would drop lower as you travelled north to Edinburgh
• It tells you the time, since the pole star's height changes through the night

Why the circles are where they are

Five reference circles get names, and four of them sit where they do for one reason: the Earth's axis is tilted at 23.5° to its orbit. • The Tropic of Cancer (23.5° N) and the Tropic of Capricorn (23.5° S) mark the furthest north and south the Sun can ever be directly overhead. That limit is the tilt, which is why the number is the same as the tilt. • The Arctic and Antarctic Circles mark where the Sun can stay below the horizon for a whole day in midwinter, and above it all day in midsummer. They sit at 90° minus the tilt, so work that out rather than memorising it. • The Equator (0°) is the only one that would exist even with no tilt at all. Tilt the axis differently and every one of those circles moves. That is worth carrying into the seasons topic.

Find the Arctic Circle

The Earth's axis is tilted at 23.5 degrees. At what latitude, in degrees north, does the Arctic Circle lie?

Match each reference point to its position

The air in the way

Starlight crosses light years of empty space unchanged and then spends its last few milliseconds being spoiled by 100 km of air. Three effects: • Sky colour. Air scatters short wavelengths most, so blue light bounces around the sky and reaches you from everywhere at once. A daylit sky drowns out every star but the Sun. • Skyglow. Wasted artificial light, thrown upwards instead of at the ground, scatters in exactly the same way and produces an artificial version of the same problem at night. This is light pollution. • Seeing. Pockets of air at different temperatures bend starlight by tiny, shifting amounts, so a point of light shimmers. Astronomers call this seeing, and it is why stars twinkle and planets, being small discs rather than points, mostly do not. Which is why the great observatories are all in the same sort of place: high on a mountain, so there is less air overhead; somewhere dry, since water vapour both absorbs and stirs; and far from any city, so there is nothing to make skyglow. Mauna Kea and the Atacama are not remote by accident.

Where would you build it?

You are choosing a site for a new optical telescope. Select the THREE features that would genuinely improve what it can see.

• High altitude, around 4000 m
• An extremely dry climate with few cloudy nights
• Far from any town or city, and from roads that carry night traffic
• On the coast, where sea breezes keep the air moving and clear
• At sea level, where the thicker air protects the mirror from temperature swings
• Near a city, so that staff and power are easy to reach

Planet Earth summary

The Earth is an oblate _____, wider through the Equator than pole to pole because it spins. Its layers from the surface inwards are crust, mantle, outer core and inner core, and we know the outer core is _____ because one kind of seismic wave cannot cross it. Your latitude equals the altitude of the celestial _____, so measuring Polaris measures where you are. The Tropics and the polar circles all sit where they do because of the Earth's axial _____.