Think about the last really good sunset you saw. The sky may have gone from pale blue to gold, orange and deep red in less than an hour. Nothing about the Sun itself changed colour. What changed was the journey its light took to reach your eyes.
Sunlight isn’t actually one colour
What looks like white sunlight is made up of many colours. Each has a different wavelength: blue and violet light have shorter wavelengths than red and orange light. When sunlight enters Earth’s atmosphere, it encounters tiny molecules of nitrogen and oxygen. Those molecules scatter shorter wavelengths much more efficiently than longer ones, a process called Rayleigh scattering.
On a clear day, blue light is scattered across the sky from countless directions. When you look away from the Sun, that scattered light is what makes the sky appear blue. Violet scatters even more strongly, but our eyes are less sensitive to it and the sunlight we receive contains less violet.
At sunset, sunlight takes the scenic route
When the Sun is high in the sky, its light travels through a comparatively short stretch of atmosphere to reach you. As the Sun nears the horizon, that route becomes much longer. Much of the blue and violet light is scattered out of the direct beam before it reaches your eyes. More of the reds and oranges remain, which is why a low Sun can look warm and dramatic.
The effect is not an optical illusion created by the sea or landscape. You can see it from a city window, a field or an aeroplane. The atmosphere is doing the work.
Why are some sunsets far more dramatic than others?
Clouds can catch the low-angle sunlight and reflect its warm colours, while dust, haze and other airborne particles alter how light is scattered. The effect is complicated: some particles can intensify colour, while thick pollution or cloud can simply block the view. A clear strip of sky near the western horizon with clouds higher up can be especially striking.
There is a lovely twist: on Mars, fine atmospheric dust can produce a bluish glow close to the setting Sun. So the familiar Earth sunset isn’t a universal rule for every planet.
A small experiment for the next clear evening
Look directly overhead while the Sun is still reasonably high, then compare that blue with the lighter colour near the horizon. As sunset approaches, watch the sky close to the Sun change colour while the upper sky remains blue. You are seeing the same physics from two different viewing directions.
The takeaway: the Sun is not changing colour. Earth’s atmosphere is sorting its light on the way to us.
Sources and further reading
NASA Space Place: Why Is the Sky Blue?
US National Weather Service: Why Is the Sky Blue?
Cover artwork is an illustration, not a documentary photograph.