Pale Blue Dot - Carl Sagan [58]
*The Earth, by definition, is 1 AU from its star, the Sun.
*Radio signals that both Voyagers detected in 1992 are thought to arise from the collision of powerful gusts of solar wind with the thin gas that lies between the stars. From the immense power of the signal (over 10 trillion watts), the distance to the heliopause can be estimated: about 100 times the Earth’s distance from the Sun. At the speed it’s leaving the Solar System, Voyager 1 might pierce the heliopause and enter interstellar space around the year 2010. If its radioactive power source is still working, news of the crossing will be radioed back to the stay-at-homes on Earth. The energy released by the collision of this shock wave with the heliopause makes it the most powerful source of radio emission in the Solar System. It makes you wonder whether even stronger shocks in other planetary systems might be detectable by our radio telescopes.
CHAPTER 10
SACRED BLACK
Deep sky is, of all visual impressions, the nearest akin to a feeling.
—SAMUEL TAYLOR COLERIDGE, NOTEBOOKS (1805)
The blue of a cloudless May morning, or the reds and oranges of a sunset at sea, have roused humans to wonder, to poetry, and to science. No matter where on Earth we live, no matter what our language, customs, or politics, we share a sky in common. Most of us expect that azure blue and would, for good reason, be stunned to wake up one sunrise to find a cloudless sky that was black or yellow or green. (Inhabitants of Los Angeles and Mexico City have grown accustomed to brown skies, and those of London and Seattle to gray ones—but even they still consider blue the planetary norm.)
And yet there are worlds with black or yellow skies, and maybe even green. The color of the sky characterizes the world. Plop me down on any planet in the Solar System; without sensing the gravity, without glimpsing the ground, let me take a quick look at the Sun and sky, and I can, I think, pretty well tell you where I am. That familiar shade of blue, interrupted here and there by fleecy white clouds, is a signature of our world. The French have an expression, sacre-bleu!, which translates roughly as “Good heavens!”* Literally, it means “sacred blue!” Indeed. If there ever is a true flag of Earth, this should be its color.
Birds fly through it, clouds are suspended in it, humans admire and routinely traverse it, light from the Sun and stars flutters through it. But what is it? What is it made of? Where does it end? How much of it is there? Where does all that blue come from? If it’s a commonplace for all humans, if it typifies our world, surely we should know something about it. What is the sky?
In August 1957, for the first time, a human being rose above the blue and looked around—when David Simons, a retired Air Force officer and a physician, became the highest human in history. Alone, he piloted a balloon to an altitude of over 100,000 feet (30 kilometers) and through his thick windows glimpsed a different sky. Now a professor at the University of California Medical School in Irvine, Dr. Simons recalls it was a dark, deep purple overhead. He had reached the transition region where the blue of ground level is being overtaken by the perfect black of space.
Since Simons’ almost forgotten flight, people of many nations have flown above the atmosphere. It is now clear from repeated and direct human (and robotic) experience that in space the daytime sky is black. The Sun shines brightly on your ship. The Earth below you is brilliantly illuminated. But the sky above is black as night.
Here is the memorable description by Yuri Gagarin of what he saw on the first spaceflight of the human species, aboard Vostok 1, on April 12, 1961:
The sky is completely black; and against the background of this black sky the stars appear somewhat brighter and more distinct. The Earth has a very characteristic, very beautiful blue halo, which is seen well when you observe the horizon. There is a smooth color