What's covered
Key facts
The Big Bang theory proposes that the universe began about 13.8 billion years ago from an extremely hot, dense point and has been expanding ever since.
Day and night are caused by the Earth rotating on its own axis once every 24 hours; the side facing the Sun experiences day, the side facing away experiences night.
The Earth takes about one year (365.25 days) to complete one orbit around the Sun; the extra quarter-day is why a leap year adds a day every four years.
The eight planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
A galaxy is a huge collection of stars, gas, and dust held together by gravity; our galaxy is the Milky Way.
Gravity is the force that keeps planets in orbit around the Sun and moons in orbit around planets; it acts at a distance and between all objects with mass.
A light-year is the distance light travels in one year (about 9.5 trillion km); it is used to express the vast distances between stars and galaxies.
Mars is known as the "red planet" because its surface is covered in iron oxide (rust) dust, giving it a reddish colour.
Seasons are caused by Earth's tilted axis (about 23.5°) as it orbits the Sun; when a hemisphere tilts toward the Sun it receives more direct sunlight and experiences summer.
Sample questions
A taste of the 10 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
Which is the closest planet to the Sun?
- •Earth
- •Mars
- ✓Mercury
- •Venus
What does the Big Bang theory propose about the universe?
- ✓It began as a hot, dense point and has expanded ever since
- •It has always existed at exactly the same size
- •It is slowly shrinking back towards a single point
- •It was formed when two galaxies collided
Which planet is known as the red planet?
- •Jupiter
- ✓Mars
- •Mercury
- •Venus
What causes day and night on Earth?
- •Earth orbits the Sun once daily
- ✓Earth spins on its axis once per day
- •The Moon blocks the Sun for half a day
- •The Sun moves around Earth
What causes the seasons?
- •Earth gets closer to or further from Sun
- ✓Earth's tilt as it orbits the Sun
- •The Moon blocks the Sun in summer
- •The Sun changes brightness during the year
Roughly how long does Earth take to orbit the Sun?
- •a single day (24 hours)
- •about ten years
- •one month (about 28 days)
- ✓one year (365.25 days)
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