“One of the problems of human knowledge is that the world which we see from the surface of this planet on a sunny day bears almost no resemblance to the Universe at large. Our Earth is made of iron and rock, but the Universe as a whole is mostly hydrogen. The actions which we see on the surface of this Earth run mostly on sunlight, but the Universe runs on gravity. What we see here are continents, oceans, rivers and lakes, mountain ranges, forests, tundra and prairies. But the Universe at large is mostly gas, partly condensed by gravity to galaxies and stars, and lightly sprinkled, here and there, with interstellar dust. The dust is made from hydrogen in the bellies of the stars, and is scattered through the galaxies by the explosions and the stellar winds of stars much bigger and much hotter than our Sun. But the dust is scarce, and, like our bodies, the rock on which we live is made of these dusts. It is a collector’s item. The heavier elements, such as iron, have sunk to the center, overlaid with the rocks of the mantle and the crust and a thin veneer of water and gas. Since the age of this museum piece is pushing five billion years, by now the water-soluble compounds of the surface rocks have leached into the water layer, making the oceans salty. The saltiness of our blood is the saltiness of the ancient sea, some four hundred million years ago. That is when our scaly ancestors, on stumpy fins, crawled out across the land in search of other water and the sight of other fish. Descended as we are from them, we can think of our bodies, even now, as little bags of sea water, hung out on clotheslines of bone, gulping oxygen directly from the gas layer above us, and shambling out across the rocks to gaze with starry eyes, through the blackness of night, at the vast expanse of the Universe beyond.
Even the oxygen that we breathe is freed by sunlight through the instrumentality of our photosynthetic relatives, first by the blue-green algae in the sea, and now by the green leaves of the rain forest. Even the rain is driven by sunlight. But the Universe at large has a reducing atmosphere, and it is without rain and without sunlight. It is very cold, very dark, and very lonely, trying desperately to fall together by the seemingly inexplicable attraction of the particles for each other. Even the radiation of the Sun is driven by this attraction which has pushed the central temperature of the Sun up to some fifteen million degrees Celsius. And it is only because its gravitational collapse has been slowed by the nuclear fusion at its core that the Sun has bathed our Earth with its warming rays for nearly five billion years. Only this delay has made possible our long genetic development till we were able to climb out of the water and gaze in wonder at the starry sky of night. Although we, as living organisms, owe both our existence and our long genetic development to the Sun, its dazzling brightness prevents us from seeing the Universe by day. The blueness of the daytime sky is not the color of the air, but simply the shorter wavelengths scattered from the sunlight by the gas layer above us. And that gas layer by night, unlit by the Sun, is sufficiently transparent so that through it we may gaze into the far reaches of the Universe.” – John Dobson.
Something to keep in mind next time you look up at the stars.