Why is the sky blue in the day and black at night?February 9, 2018
Why is the sky blue?” is probably one of the first questions a child asks their parent, followed by, “So why is the sky it dark at night?” Both questions have tricky answers, and the latter is famously known as “Olber’s Paradox“.
Why is the sky blue?
In general, light travels in a straight line unless if it is either reflected, bent, or scattered. White light from the Sun reaches Earth’s atmosphere and is scattered in all directions by gases (composed mainly of nitrogen and oxygen) and particles in the air. The light interacts with the molecules and particles and the blue light is scattered. So when you look up into the atmosphere, you are actually looking up at the scattered blue light.
When you look out to the horizon, the sky fades to a lighter blue (or even white) because the Sun’s light is low in the sky and has passed through even more atmosphere than the sunlight reaching your eyes from overhead. When you look more-or-less directly at the Sun, the light you are seeing has had most of its blue light scattered, leaving the light you are seeing yellowish in color (so the Sun itself appears yellow).
At sunrise or sunset, the light travels though the atmosphere at a low angle and passes through much more atmosphere, so the only light that gets through is red – the colors of light with shorter wavelengths are scattered. If there is lots of dust in the atmosphere, then there are more particles in the atmosphere to scatter the light so the sunsets appear even more red.
In space, there are no air molecules or dust particles to scatter the different colors in light, so the Sun looks white and the ‘sky’ looks black. This is why images from the space station or shuttle always show the sky as “black”, and the Sun “white”.
So why does blue light scatter?
Blue light scatters more easily because it has a much shorter wavelength than, say, red light. The other colors have wavelengths in between. Red light’s wavelength is significantly longer than the size of the particles and molecules in the atmosphere. Imagine a small post in a harbor and an ocean swell with a long wavelength coming in: the post will be too small to make much of a reflection (the difference in wave height between the two edges of the post is tiny so there is little to reflect). However, if there is a much smaller wave coming in to the harbor (a “blue” wave) with a short wavelength that is close to the size of the post, then there will be a much greater reflection. Going back to our sunrise/sunset example, it follows that since red light has the longest wavelength of the visible colors, it has the greatest chance to be “seen” after passing through the most atmosphere (see diagram above), giving sunrise/sunset their red colors.
Why is the sky black at night?
This question is actually much more complicated than it seems. Heinrich Wilhelm Olbers is known for proposing this question in the 1820’s. His question is known as Olber’s Paradox: if the universe is endless (infinite) and is full of bright stars, then every line of sight must end at the surface of a star, and the sky should be completely bright with white light. But, it isn’t – the sky is dark at night, almost black.
The answer is not trivial, and it was only with the advent of Einstein’s Theory of Relativity and Hubble’s discovery that objects in the universe are moving away from one another, that scientists were able to propose a reasonable solution.
Perhaps dust scattered throughout the universe is obscuring the light? This is not correct because any dust in the path of the light would eventually re-radiate and heat up, producing light, and making the night sky bright.
Perhaps the light is lost due to the extreme distances to the stars? This is not correct either, because we know that although the amount of light to reach us from each star decreases with distance (by 1/d^2), the number of stars in each layer increases, effectively balancing out the distance effect.
So what’s the deal? Most modern cosmologists have settled on two theories to account for the darkness.
The first one states that an object’s red shift – a doppler-like effect that tells us objects are moving away from us – indicates that space itself is expanding, and thus decreases the amount of light reaching our eyes.
The most generally accepted theory is that the universe is not infinitely old – it has a beginning point. If the universe were infinitely old, the sky would in fact be infinitely bright, because light from every point in the universe would have had an infinite amount of time to travel to every other point. But this is clearly not the case. So it is therefore the finite age of the universe limits how much light we see, and why the sky is dark at night.
Confused? Well, you’re certainly not the only one! This video will surely help to explain…