The Solar System · revision map · IGCSE 0625, Topic 6.1

The Earth spins on its axis and orbits the Sun; the Moon orbits the Earth.

• One spin on its axis takes a day.
• One orbit of the Sun takes a year.
• The Moon orbits in about a month; we see phases as sunlight lights different parts of it.
• The tilt of the axis gives the seasons.

The Sun and eight planets, held together by the Sun's gravity.

• Four small rocky planets lie closest in; four large gas and ice giants lie further out.
• There are also moons, asteroids and comets.

Gravitational attraction provides the force that holds objects in their orbits.

• The pull is toward the centre of the orbit.
• It is weaker at greater distances.
• Orbits are slightly elliptical, not exact circles.

The Solar System formed from a rotating cloud of gas and dust.

• Gravity pulled the cloud together; most mass gathered to form the Sun.
• The rest formed a spinning disc, where matter stuck together (accreted) into the planets.
• The Sun holds almost all of the mass.

The eight planets fall into two groups by size and make-up.