Gravity and orbits
Earth is one of eight planets orbiting the Sun, which is one star among billions in our galaxy, the Milky Way. Gravity holds planets and moons in their orbits. Day and night come from Earth spinning; a year is one orbit of the Sun.
Key facts
- Earth spins once a day (day and night) and orbits the Sun once a year.
- Seasons happen because Earth’s axis is tilted.
- The Moon orbits Earth about once a month; we see it by reflected sunlight.
- A light year is the distance light travels in a year.
Worked examples
Example 1
Light takes 8 minutes to reach Earth from the Sun. Light travels 300 000 km each second. About how far away is the Sun?
- Minutes to seconds. 8 × 60 = 480 s
- Multiply by the speed. 480 × 300 000 = 144 000 000 km
Answer: 144 million km
Example 2
Why is it summer in the UK when the northern half of Earth tilts towards the Sun?
- The northern half leans towards the Sun.
- The Sun climbs higher in the sky, so its light is more concentrated.
- Days are longer, so there is more time to heat up.
Answer: The Sun is higher and days are longer