The shape and size of the Earth

The Earth is very nearly a sphere — strictly an oblate spheroid, because its rotation makes it bulge slightly at the equator and flatten at the poles. Its mean radius is about 6371 km, so its circumference is roughly 40 000 km.

• Ships disappear hull first over the horizon.
• The Earth casts a circular shadow on the Moon at every lunar eclipse.
• Different constellations are visible from different latitudes, and the altitude of Polaris changes as you travel north or south.
• Photographs from spacecraft show the disc directly.

Exam tip: "photos from space" alone is a weak answer. Edexcel wants the historical, ground-based evidence too — the circular shadow in a lunar eclipse and the changing altitude of Polaris are the classic marks.

Eratosthenes measures the Earth

Around 240 BC Eratosthenes knew that at noon on the summer solstice the Sun was directly overhead at Syene (it lit the bottom of a well), while at Alexandria — almost due north — a vertical stick cast a shadow showing the Sun was 7.2° from vertical.

The assumptions: the Sun is so far away that its rays arrive parallel, and the two cities lie on the same meridian (same longitude).

Your turn — the size of the Earth

Latitude and longitude

Any point on the Earth is fixed by two angles measured from the centre:

• Latitude — degrees north or south of the equator (0° at the equator, 90° at the poles).
• Longitude — degrees east or west of the Greenwich (prime) meridian, up to 180°.

Longitude and time are the same idea in different units. The Earth turns 360° in 24 hours, so:

Useful trick: the altitude of Polaris above your northern horizon is (very nearly) equal to your latitude. From London (51.5° N) Polaris sits about 51.5° up; from the equator it sits on the horizon.

Your turn — longitude and time

Which way does it spin?

Rotation: day and night

The Earth spins once on its axis, so half of it faces the Sun (day) and half faces away (night). Astronomers use two different "days":

• Solar day — noon to noon, i.e. Sun to Sun. Averaged over the year this is exactly 24 h.
• Sidereal day — one full 360° turn measured against the stars. It lasts 23 h 56 min 4 s.

The sidereal day is about 4 minutes shorter because, while the Earth spins, it has also moved about 1° along its orbit — so it must turn roughly 1° extra to bring the Sun back to the meridian.

Consequence: the stars rise about 4 minutes earlier each night (2 hours earlier each month). That is why the constellations you see change through the year.

Axial tilt and the seasons

The Earth's axis is tilted 23.5° to the perpendicular of its orbit, and it always points the same way in space (towards Polaris). As the Earth orbits, first one hemisphere then the other leans towards the Sun.

Killer misconception: the seasons are not caused by the Earth being nearer the Sun. The Earth is actually closest to the Sun in early January — northern midwinter. It is the tilt (Sun's altitude + length of day) that matters.

Your turn — the noon Sun

Why the seasons?

How the atmosphere spoils the view

Everything you observe from the ground is seen through about 100 km of moving, dirty air. Edexcel expects you to know the problems it causes:

• Scintillation ("twinkling") — turbulent pockets of air of different density refract starlight, so a star's image dances and flickers. (Planets show a small disc, so their light averages out and they twinkle far less — a useful way to tell a planet from a star.)
• Atmospheric extinction — the air absorbs and scatters starlight, dimming objects. It is worst near the horizon, where you look through the most air, and it makes objects look redder.
• Refraction — the air bends light, so objects appear slightly higher than they really are; the Sun is still visible for a couple of minutes after it has actually set.
• Light pollution and cloud — city skyglow drowns out faint objects.
• The atmosphere blocks most X-rays, ultraviolet and infrared, so those telescopes must go into space.

Which is why: professional observatories sit on high, dry mountains far from cities (thin air, little water vapour, dark skies) — and the very best "seeing" of all is from orbit.

Twinkle, twinkle

What is that caused by?

Tap an observation, then tap the property of the Earth that explains it.

Match the description to the term

Tap a description on the left, then its matching term on the right.

Finding your latitude

The latitudes the tilt creates

The 23.5° tilt draws four special circles on the globe:

• Tropic of Cancer (23.5° N) and Tropic of Capricorn (23.5° S) — the furthest north and south the Sun can ever be directly overhead at noon (at the June and December solstices).
• Arctic Circle (66.5° N) and Antarctic Circle (66.5° S) — poleward of these the Sun stays above the horizon for 24 hours at the summer solstice (the midnight Sun) and never rises at all at the winter solstice.

The four key dates: the equinoxes (around 21 March and 23 September) — the Sun is over the equator and day and night are equal everywhere; and the solstices (around 21 June and 21 December) — the Sun is over a tropic and the days are longest or shortest.

The midnight Sun

The big ideas to know

That is Planet Earth covered for Edexcel GCSE Astronomy. Press Finish to see your score.