Why is The Night Sky Black

To find out, we need to understand Olbers’ Paradox. In the early-20th Century, astronomers had a problem.

It was largely believed that, in both space and time, the universe was infinite and non-moving. Under Newton’s law of gravitation, this was t he only way the universe could be seen to be stable.

However, a German astronomer called Heinrich Wilhelm Olbers had come up with a problem in 1823 that seemed unsolvable.

If stars and galaxies were randomly spread throughout an infinite, static universe that had been around forever, then at every point in the night sky we should see a star emitting light. Known as Olbers’ Paradox, this conundrum suggested that the night sky should be uniformly bright, not dark.

Astronomers started observing stars and galaxies and, instead of them being static, they realised that everything was actually moving away from one another.

They measured this by noting the red shift of stars, the movement of light towards one end of the spectrum as a source moves away (like a police siren moving away from you that spreads out its sound waves and appears to lower in pitch).

They realised that the universe was expanding from a Big Bang, and thus Olbers’ Paradox was satisfied. With a finite, non-static universe, it was deduced that not only had the light from every star not reached Earth – as they each had a finite lifetime – but also there was almost certainly not a star in every direction, meaning the night sky should appear black.