White light, like sunlight, is composed of all the colors of the rainbow. And if you saw a sunset on the moon, the sun would look white. But Earth has an atmosphere. And tiny molecules in our atmosphere cause light to scatter. That’s why our sky looks blue: it’s because sunlight scatters the bluish component of white sunlight. And it’s why the sun looks reddish when it’s near the horizon.

Think about what happens as the sun sinks lower in the sky. Its light has to travel farther and farther through the atmosphere before reaching your eyes, simply because there’s more atmosphere in the direction toward the horizon than overhead.

Sunlight encounters more air molecules when the sun is low in the sky than when the sun is overhead. Even more blue light is scattered away, leaving mostly the reddish component of white sunlight to travel the straighter path to your eyes. So the setting sun looks red.

People used to comment that autumn sunsets were more spectacular than any other. In many places, that time of year typically brought clear weather, so you’d have a good chance of seeing a beautiful sunset. Now, with the climate changing, who knows?