If you’ve ever looked up into the sky during the night, and wondered why the sky is dark, even though there are infinite numbers of stars? Also though there is a moon illuminating with bright light? Have you ever questioned this? Yes like us, this question has been posed by many philosophers and physicists over a thousand years, from the Ancient Greeks to the twentieth-century astronomers.
It is one of the ancient puzzles in the astronomy, but the answer is not only relatively simple, but it’s also rather illuminating.
Why The Sky Is Dark At Night?
The question about “Why the sky is dark at night?” It was popularised in the 19th century by a German Physician called Heinrich Olbers, who is a doctor during the day and an astronomer at night. While many others had spoken about this problem before, Olbers came up with a paradox which was named after him: “Olbers’ paradox,” also known as the “dark sky paradox” to answer this question.
You may be thinking there is a simple answer to this question, and surely the night sky is dark because the Sun has left in the night hours? Is it true? Let’s see
What did our ancestors consider?
Think back to what our previous generations have been considering about the Universe. The Sun, the planets, the Moon and the stars which are in the sky, would rise again after setting. These things were known, and they were constant.
Based on some of these observations, our ancestors drew the following conclusions about the Universe:
- That it was the same in all directions because you see stars in every course you look at, (we call this a homogeneous Universe).
- That it was unchanging, forever remaining the same because nothing changed with each passing year (a static Universe)
- That the Universe was in the night because as telescopes grew with the centuries, an ever-growing number of fainter stars were found in every part of the sky. If all these things about the Universe are correct, then every line of sight, every single place you look in space should eventually become a star.
The sky is dark at night because,
There are two things here to think about. Let’s take the easy one first and ask, “Why is the daytime sky blue on Earth?” That is a question which we can answer. The daytime sky is blue because the light from nearby Sun hits the molecules present in Earth’s atmosphere and scatters off in all the possible directions. The blue colour of the sky is a result of this scattering. During the night, when that part of Earth is facing away from our Sun, the sky looks black because there is no light source of light, like the Sun, to be scattered. If you were on the Moon, which has no atmosphere, the sky would be black both at night and day. You can see this in photographs taken during the Apollo Moon landings.
So, now coming to the harder part – if the Universe is full of stars, why doesn’t the light from all of them sum up to make the whole sky bright all the time? It turns out that if the Universe were infinitely large and infinitely old, then we would expect the night sky to be illuminated from the light of all the stars. Every direction you look into space, you would be looking at a star. Yet we know from experience that area is pitch black. This paradox is known as Olbers’ Paradox. It is called a paradox because of the apparent contradiction between our experience that night sky is black and expectation that the night sky is bright.
Many different explanations have been put across to resolve Olbers’ Paradox. The best solution is that the Universe is not old; it is somewhere around 13 billion years old. Which means we can only see objects which are as far away as the distance light can travel in 13 billion years. The light from stars which are far away than that doesn’t have time to reach us due to which they can’t contribute to making the sky bright.
Another reason why the sky may not be bright with the visible light of all the stars is that when a source of light is moving away from you, the wavelength of that light is made longer (which for view means more red.) This means that the light from the stars that are moving away from us will become shifted towards red and are no longer visible to us.