For the vastly greater part of the experience of humans on Earth, we have perceived the firmament as curved. At night it appears as a black dome with points of starlight, roving planets, comets, and meteors stuck, as it were, to its inside surface. The daytime sky viewed by our ancestors (and unthinking, ourselves) appears as a blue bowl-shaped ceiling with cloud formations hovering from the large high ones at the zenith to the small low ones hugging the horizon.In the image above, we view the clouds above us as large and high. Toward the middle of the scene, layers of clouds appear smaller and lower. Those clouds near the horizon appear very small and low to the ground.The same visual effects can be viewed in the image below this text. Large high clouds near us and small low clouds at the limit of our vision.In a cognitive way, we understand that the sky is not a dome but it is a limitless expanse. We also know that cloud layers are quite flat – as a result of atmospheric conditions that tend to organize them into sheets of vapor whose different heights are sustained by meteorological circumstances, such as wind and temperature. This fact communicates its strongest expression to those sitting in the window seats of jetliners. Passengers see the clouds in their proper aspect of relatively flat layers of vapor. We are therefore, in a sort of sandwich between the sheets of sky above us and the earth below our feet.The existential disconnect and cognitive dissonance we experience as regards the shape of the sky is a common and ancient one. On the one hand, we experience things in a particular way and on the other hand, we know many things are different than they may appear. Nevertheless, our habit is to experience the world as it appears to us and not how we know it to be – factually.By now we have an easy familiarity with the railroad track illusion, in which we observe the factually parallel tracks as coming together and meeting at the horizon. This is an obvious instance where we know the tracks to be parallel but we experience them only as if they meet at a distance. Our perception overrides our cognition. We cannot see things as we know them to be. We can only experience their appearance.The lines above illustrate the nature of our perception when it comes to viewing things on the ground – such as railroad tracks. We seem to “get it” that things near us appear larger than the same things would appear when placed at some distance before us. A more fully diagrammed perspective map of our apparent field of vision is above. The red horizon line in the middle of the image holds the “vanishing point” – a virtual point directly before us toward which our entire field of vision converges.As this description moves along it becomes more clear as regards our field of vision that what factors are at work in our perception of railroad tracks are also at work in our perception of things floating in the sky.And so it becomes easier to “see” that the clouds above us are not hung from top to bottom but are displayed across our field of vision distorted by perspective. When accompanied by rays of sunlight the illusion is evident. The spreading rays of light are an artifact of that aspect of electromagnetic radiation, which causes it to spread out in a conical manner as it moves outward from its source.With an increased awareness of the often illusory nature of experience, we become slightly more able to perceive the sky as we know it now to be. But this is often a fleeting phenomenon. There is a great tendency to unconsciously drift back to the more familiar illusion. It takes some daily effort to attempt to rectify the relationship between appearance and reality. We are able to see more clearly now. But chances are tomorrow we will forget.