Jumping up and down in his ecstasy he beams

into the empty blue, eastward, over the parapet

toward the city  .  .

—Paterson, William Carlos Williams

CHILDREN, WITH THEIR KNACK for asking the best questions, often ask why the sky is blue, since air is colorless. Parents who struggle to answer this question can take comfort in knowing that it also troubled Aristotle, Descartes, and Newton (who was so interested in colors that he stuck thick needles between his eyeballs and his sockets, in order to trigger trauma-induced blooms of them). By the nineteenth century the origin of the sky’s color was one of the more engaging and popular questions in science, crisscrossing disciplines and catching the attention of philosophers, poets, and artists, too.

A short explanation: When sunlight encounters the molecules that make up the air, some of the light bounces off, or scatters. Shorter-wavelength colors, like blue, are scattered much more effectively. The remaining sunlight that reaches our eyes is slightly yellowed by the loss of some of its blue, while the sky, filled with that scattered blueness, takes on the hue to which sapphires have so often been compared.

When the sun is low in the sky, its light must pass through much more air to reach our eyes. This means even more of its blue is scattered, along with other colors, leaving the predominantly red hues we associate with sunrise. (That is to say, when we look up at the blue of the daytime sky, we see the color that has been scattered; when we look toward the dawn, we see the colors that have not.) The greater ability of red light to travel through air is one reason the color is so well suited to traffic lights, for example, and to the steady beacons mounted on tall buildings for the purpose of warning off aircraft at night.

Often, through the wide windows of the cockpit, when the western sky is black, but red has started to burn in the east, a stunning spectrum of blues slowly asserts itself across all the sky in between: from a pale, almost gray shade through cobalt to a deep midnight blue still shot through with stars. Sometimes these blues flow continuously from one into another, and alter nearly imperceptibly as our gaze moves across the sky. On other mornings, especially ones experienced from within the icy clarity of higher altitudes, the blues seem to be arranged in distinct stripes spread across the sky; or perhaps that’s only the brain attempting to segment what is, in nature, infinitely varied.

Later, in the brightness of day, it can be hard to remember that the stars have not been washed out by the sun. (In space you could be in direct sunlight and still see a night-like arrangement of stars shining out of a black sky.) Rather, in the daytime, the stars are obscured by the brightness of the blues that the particular components and proportions of our atmosphere scatter from the sun’s light. Indeed, billions of years ago, and before the development of life itself altered it, the composition of the atmosphere was different, and so was the sky’s color—perhaps pale white or yellow, writes Götz Hoeppe, in Why the Sky Is Blue.

Finally, in the evening, at what is called the blue hour, the fading light steadily disassembles the high day’s vivid blues until their final disintegration reveals the stars again, along with, on a moonlit night, some of the most subtle blues of all. Even when I was a little kid I loved dark blue, and according to one of Mom’s favorite stories about me I used to often shout: I don’t want the sun to shine. I want the moon to shine! Only as an adult did I learn that the midnight blue of a moonlit sky has the same origin as the brilliant cerulean of a sunny day—for the moon’s light, of course, is only the sun’s. It’s a truth that reminds me that not every planet is blessed with blue skies, and that a love of the color might arise naturally, and at any hour, in those who dream of crossing them.