The Shark and the Jellyfish: More Stories in Natural History

The Light Fantastic

165 The Light Fantastic We live beneath a special sky—a compromise between extremes. Our atmosphere is unique in the solar system, neither an oppressive overcast (like that of Venus or Titan) nor a tenuous near vacuum (like the deep space close upon the surfaces of Mars or Europa). The layer of air that covers our Earth is substantial enough to shield us from the lethal winds of the cosmos, yet it is not opaque—it is transparent. So while our bodies are protected by the blanket of air that covers us, our mind’s eye is free to roam among the lights of the night—to rise up and then look back down upon Earth from the perspective of its starry context. Looking up, we see the lights of the stars through the sea of swirling air currents miles deep that stirs the air above us. The farther from the zenith we look, the deeper the atmosphere is—forty times more air intervenes when our sight wanders just above the horizon than when we glance straight up. The currents in the ocean of air constantly mingle pockets of different wind speeds and temperatures. Buoyant updrafts fly past chill zephyrs spilling from the stratosphere; eddies spin where zones of wind shear moan against one another. When the light from the stars crosses boundaries between these different regions of air, its path is bent. Sirius, the Dog Star, stands out among the night’s lights as a focal point for our sense of celestial wonder—the brightest star in the night sky. Its light is refracted and reflected from the boundaries between the different pockets of air just as it would be by the solid faces of a prism, though not as strongly. Each of these air pockets acts as a miniscule lens. Their effects are amplified by their vast numbers—even more so when a star is low in the sky where the intervening air is thicker. Fifty miles of atmosphere surges between our eye and the image of Sirius rising just above the horizon. The fleeting lenses in the air flicker in and out of existence, animating the light of the stars—causing them to sparkle. The effect is greatest on cold, crisp evenings in the hours when the air still roils with the energy of the day as the stars are first appearing. Early in February, Sirius is climbing just above the murk of the southeastern horizon and is already at full brightness when darkness asserts itself. Sirius’s shallow rising angle keeps 166 it in the low region of greatest atmospheric effect longer than any other of the bright stars, which rise more vertically toward their zeniths. Seen through a small telescope at dusk on the winter cross-quarter day (halfway between the solstice and the vernal equinox, about February 5), Sirius appears to swim in and out of focus. Its light dances like the image of a sunlit coin shining up from the bottom of a stream, distorted by the ripples on the surface. Seen on that day with the naked eye, Sirius the evening star scintillates —its gleam in constant motion. Hovering in the southeast, where the backdrop darkens early under the arc of the rising shadow of the Earth, the Dog Star comes alive, winking to catch our attention. Though the air is never more transparent than through storm-washed late-winter skies, the light of this restless star sometimes disappears, switched off for a split second—then it comes back and sparkles even brighter than its true brilliance. Suddenly the star flashes electric blue for an instant; a moment later, it flashes with a pulse of ruby red. Our atmosphere is dividing the starlight, parsing the distant beam into darts of color. These are not stellar hues—not the pale yellow of Capella, not the Martian orange cast of Betelgeuse. No star is green; no visible star is bright red. These are bold prismatic colors— the same pure shades we see refracted from dewdrops when the morning sunlight hits them just right—but with an emphasis on the blue. Sirius is a blue star, actually brightest in the ultraviolet wavelengths our eyes cannot see. So its sapphire scintillae are especially prominent—a glittering counterpoint to the deepening gray of a late-winter evening. Those rays of starlight have been traveling at the speed of light for the last eight years, across trillions of miles of space. Yet the sparkle in their luster is...