She told him stories. He taught her to fly.

(A table of contents will be available at the end of the series. In the meantime, you can find previous installments in the space building tag and other original posts in the posts by pear tag.)

Part Thirteen: Look to the Skies!

One popular feature of sci-fi stories taking place on constructed worlds is a sky of a different color. It’s important to understand the basic mechanics of why the sky is the color it is on Earth so that you can adjust it for your own planet in a way that might be plausible.

Why is the sky blue?

You may have heard the myth that the sky is blue because it reflects the ocean. This, of course, isn’t true. Instead, the reason the sky appears blue is because of how our atmosphere’s particles scatter light and how much atmosphere the light must pass through in order to reach us.

If you’ve taken any science classes in school, you’ll remember that light is split up into a spectrum of coloration, and each color vibrates at a different wavelength:

Shorter wavelengths get scattered in the atmosphere easier, breaking them apart from the main “beam” of sunlight. In Earth’s case, that scattered wavelength is blue. As the source of the light gets closer to the horizon, more particles are present in things like dust and pollution, which scatters the blue light so much that we don’t really see it anymore. Instead, we get the longer wavelength colors like red, orange, and yellow as sunset.

What impacts the coloration?

How dense is the atmosphere and with what? We talked about last time how the coloration of a planet from the outside mostly depends on what the atmosphere is composed of. That composition will also affect what types of wavelengths are scattered. Different elements will have different sizes and their composition will help determine what kinds of light will get through and which will scatter enough not to be seen much.

Some types of gases in the atmosphere will have colors of their own that will contribute.

In addition, the more dense the atmosphere, the more the shorter wavelengths will be scattered, and the more likely it will be that only the longest wavelengths will make it through to the eyes of those on the ground. Venus has a particularly dense atmosphere; pictures suggest that it has an orange glow to its sky, which aligns with the longer wavelengths being the only ones to make it through.

The less dense the atmosphere, the more the particles on the surface will be whipped up into the air, less protected and more prone to the winds of the system. Those particles and dusts and whatnot will have their own color based on their composition, and that’s the color that will be more prominent. Think Mars, here.

What color is the star? Our own star is a yellow star, which appears toward the middle of the spectrum in terms of wavelength, meaning that the colors surrounding it also come with it, and that covers pretty much all of the visible light spectrum. A star that’s emitting a different wavelength such as a red giant, would have a higher concentration of different wavelengths of light, making the dominant color on the planet likely to be slightly different.

Consider a habitable planet similar to Earth orbiting a red giant. It’s likely that the star is already mostly putting out longer wavelengths. Even with a similar atmospheric composition and density to Earth, there simply isn’t that much of the shorter wavelengths to begin with, so they scatter too much. The ones that are most likely to scatter and still be seen are probably going to be greens and yellows. Speculation, but, there you are. Principles at work.