Discover the moon's secrets during Hong Kong's total lunar eclipse on September 8
Some scientists think the moon formed about 4.5 billion years ago after a collision between an early version of Earth and a small planet called Theia. It released a huge amount of debris, eventually forming the moon we see today.
This is known as the giant-impact hypothesis.
What's on the moon?
Decades of research have found that the moon has layers. On the outside is a crust. A mantle is below that, and a core is at the centre (see graphic).
Dark skies all day
When sunlight reaches Earth, it is scattered by gases and particles in Earth's atmosphere. Blue light is scattered more than other colours because of its short wavelength. That is why our sky looks blue.
However, the moon has only a thin layer of air. Even when it is daytime there, the sky looks black. Because the sky is pitch-black, you can see the stars during the day.
Lunar eclipse
A lunar eclipse happens when the Earth is between the moon and the sun, blocking the sunlight that usually shines on the moon. This makes its surface appear darker.
A total lunar eclipse can be seen from Hong Kong on September 8. If you miss it, you will have to wait another three years for the next one.
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