Day, night and the year

The Earth has two key motions, and they explain the rhythms of our sky:

• The Earth rotates on its axis once in about 24 hours — this gives day and night and the apparent daily motion of the Sun and stars across the sky (rising in the east, setting in the west).
• The Earth orbits the Sun once in about 365 days (one year). Combined with the tilted axis, this orbit produces the seasons and the seasonal change in the Sun's path.
• The Moon orbits the Earth once in about one month.

Watch out: the Sun and stars only appear to move across the sky. It is the Earth that is rotating — the apparent motion is the reverse of the Earth's spin.

What causes the seasons?

What's in the Solar System

The Solar System is everything held by the Sun's gravity. You need to know its components and the order of the eight planets:

• The Sun — our one star, holding almost all the mass.
• The eight planets, in order outward: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
• Dwarf planets (e.g. Pluto) and asteroids (rocky, mostly in the belt between Mars and Jupiter).
• Moons (natural satellites that orbit planets) and comets (icy bodies on long, stretched orbits).

Order the inner planets

Tap the four inner planets in order, starting nearest the Sun.

Gravity holds the orbit

The Sun is so massive (about 99.8% of the Solar System's mass) that its gravitational attraction reaches out and pulls each planet. This gravitational force provides the inward (centripetal) force that keeps a planet curving around the Sun instead of flying off in a straight line.

Misconception buster: there is no mysterious "outward" force throwing the planet out. Gravity is the only force, and it acts as the centripetal force — continuously bending the planet's straight-line motion into a curve.

Why doesn't a planet fly off?

Average orbital speed Supplement

For an object in a (roughly) circular orbit, the distance covered in one full orbit is the circumference, 2πr. Dividing by the time for one orbit — the period T — gives the average speed:

Because a larger orbit also has a longer period, planets further from the Sun move more slowly: Mercury races round, Neptune crawls.

Units matter: radius in metres, period in seconds. If a question gives days or km, convert first (1 day = 86 400 s).

Your turn — orbital speed Supplement

Rearranging v = 2πr/T Supplement

Comets and the Sun's field Supplement

The Sun's gravitational field strength decreases with distance — gravity gets weaker the further out you go. This shapes how objects on stretched orbits move.

A comet follows a long elliptical orbit, with the Sun off-centre. By conservation of energy:

• Close to the Sun the field is strong, gravitational potential energy is low, so the comet's kinetic energy is high — it moves fastest.
• Far from the Sun the field is weak, potential energy is high, so kinetic energy is low — it moves slowest.

Comet speed Supplement

The Sun is a star

Our Sun is a star of medium size. Like other stars it is made mostly of hydrogen and helium.

Its energy source is nuclear fusion: in the core, hydrogen nuclei fuse together to form helium, releasing huge amounts of energy as electromagnetic radiation (infrared, visible light and ultraviolet).

Galaxies and the light-year

A galaxy is a huge collection of many billions of stars. Our Sun is just one star in the Milky Way galaxy — and the Milky Way is just one of many billions of galaxies that make up the Universe.

Distances between stars are so vast that we use the light-year:

Misconception buster: a light-year is a distance, not a time. "The star is 4 light-years away" means light needs 4 years to cross that gap — but the light-year itself measures how far, like a very long ruler.

What is a light-year?

Your turn — light-year reasoning

The life cycle of a star Supplement

Every star begins the same way, then its fate depends on its mass. All stars start as a nebula (a cloud of gas and dust) that gravity pulls together into a protostar, which heats until fusion begins and it becomes a stable (main-sequence) star. Then the two branches split:

Order a massive star's life Supplement

Tap the stages of a much more massive star in order, from birth to final remnant.

Redshift

When we look at light from distant galaxies, the lines in its spectrum are shifted towards the red (longer-wavelength) end. This is redshift: the observed wavelength has increased because the galaxy is moving away from us.

Almost all galaxies are redshifted, and the more distant a galaxy is, the greater its redshift (the faster it recedes). This is the key evidence that the Universe is expanding, supporting the Big Bang theory. Supplement Further evidence comes from the microwave background radiation — radiation reaching us uniformly from all directions, the stretched-out "afterglow" of the early hot Universe.

Misconception buster: redshift here is mainly space itself stretching the wavelengths as the Universe expands — the galaxies aren't ploughing through space like cars. And greater distance means greater recession speed, not a longer light travel time alone.

Reading the redshift

The Hubble constant and the age of the Universe Supplement

The recession speed v of a galaxy is proportional to its distance d. The constant of proportionality is the Hubble constant H₀:

If the expansion rate has stayed roughly constant, then running the expansion backwards gives an estimate for the age of the Universe:

Your turn — age of the Universe Supplement

The key ideas to know

You've covered both halves of Cambridge IGCSE Topic 6 — the Earth & the Solar System and Stars & the Universe. Press Finish to see your score.