“Why is the sky blue?” is the quintessential child’s question. Adults could query “Why is the air clear?” or “Why can we see?” or even “Why can we breathe?” Short scientific explanations about blueness refer to scattering of light by molecules. Details about which molecules are most involved on earth (nitrogen and oxygen) and why humans have evolved to perceive blue and use oxygen are usually omitted. On other planets, we now surmise, the sky might not be blue at all. Even on our own turf, different eyes on different creatures will perceive it much differently (if at all) than as a “blue experience.”
With many important things on our minds, we usually pay no attention to the sky and air around us. Perhaps a sunrise or sunset captures a moment of reflective beauty. Perhaps a storm interrupts our well-laid plans. Pollution _ local or global _ may cause indignation, but most of us happily step into the trees at a park and glance at a blue sky; smile and look away. No rain, bright sun, things are good. The way it always was, and always should be.
Early science fiction was really about society, but it tried to avoid pure fantasy. So for the most part almost all extraterrestrial stories _ from space opera to carefully crafted parlor mysteries _ took a blue sky as given. Intelligence, it was supposed, required certain normalities, the atmosphere being one. In doing so, it was simply following ancient and medieval convention.
Until recently, air and blue sky just were, filling the universe wherever there was not solid land or water. Sophisticated ancient cosmologies imagined crystal spheres floating on air, Medieval European visions assumed breathable atmosphere continuously from heaven to hell, early science figured air filled all the empty space around the moon, sun, and planets. Eventually, space was filled with ether, then vacuum. Discovery that atmosphere is a thin shell around our sphere projected that other celestial bodies must also be surrounded similarly. Only in the last hundred years were we informed that some planets, stars, moons and asteroids have none, some have superheated noxious gases, some have cold dry wind lacking oxygen, and some have nearly unimaginable mixes of peculiar gasses. Few, so far, indicate an oxygen/nitrogen composition.
Most air is nitrogen, but most effects of air are from water. Extraterrestrial searches seek “goldilocks” planets _ not too big, not too small, just the right distance from a sun to provide liquid water and gravity to keep it in place. Meanwhile, geologists increasingly believe our aquatic envelope has been provided by accidents like the moon’s creation, or just enough volcanic action to provide an aerosol sealant, or a molten core to generate a protective magnetic shield, and other exotic local accidents and solar cycles. Water may not be nearly as common a component even at “perfect” planets as we like to imagine.
And, of course, that all-important oxygen, which is only there because of just the right type of bacteria doing their thing for billions of years. Incidentally providing a protective ozone shield for just-enough mutations leading to a relatively mild evolution without radiation poisoning. Another just-right consideration. Without that, blue sky pretty much vanishes, not to mention anyone like you to see and appreciate it during a lunch walk.
Do you ever consider how it stays the way it is? How do water and oxygen keep heading back up and down. People point to trees, but it is probably really viruses and bacteria and fungi, many in the oceans, that keep our oxygen level as it is. It is waves and evaporation that keep water in the air. It is an ozone shell that helps trap the water which floats towards space, a van-allen belt that keeps hydrogen from being stripped and lost. There are so many complexities to maintenance of our sky that the mind boggles, even before we consider how many billions of years it has been relatively stable.
Every breath we take is an infinitely convoluted procedure to achieve continued existence. Each glance we make into a blue sky is an insanely moderated chain of chemical and electrical reactions that somehow gives us experience from photons. Every memory we have of storm or cloud or clear sky is impossible to comprehend as neurological storage.
Most amazingly, we do not have to understand any of it to appreciate it. Surely the earliest humans enjoyed blue sky as much as we do, at least unless they were hoping for rain. So what if, like many other things, it is insanely more complicated than we think. A child can appreciate it, with or without annoying wonder, and so can we.