From this little rock that we call home, we reliably witness the complex face of our nearest celestial neighbor, the Moon. Beyond it, orbiting the solar licks of the sun, are our fellow planets and rocky debris. Past our solar system, nearby stars and nebulae and black holes whirl within the Milky Way. Other galaxies—Andromeda, Triangulum, the Large Magellanic Cloud—are relatively close, but the scope of our universe soon explodes in magnitude. For billions of light-years in every direction we look, stunning clusters and walls and voids fill the cosmic plane. A single snapshot of our sky, a fraction of the angular size of our moon, boasts thousands of galaxies, each holding billions of stars like our own.
We have not always seen so much.
The Pleiades appear in a 30,000-year-old drawing in the Lascaux caves; 5,000 years ago, a supernova was carved into a rock in Kashmir. We find explanations of the Milky Way, once dominant in the average night sky, throughout ancient mythologies. Within our own system, the planets you might yourself observe at night—Mercury, Venus, Mars, Jupiter, and Saturn—have been identified since the astronomers of ancient Babylon. Such are the lights that reach our unaided observation, our fundamental ability to locate ourselves in the vast expanse of the universe.
Then came Galileo and optical telescopes, Soviet probes and the Voyager missions, Hubble and radio telescopes. Over a few centuries, human comprehension of the universe has expanded beyond imagination. For the past 20 years, the Sloan Digital Sky Survey (SDSS) has gathered data on more than three million galaxies and quasars (ancient, super-luminous galactic cores), resulting in one of the most ambitious cosmological data visualizations yet created: a three-dimensional map of the sky.
The “sky,” in astronomical terms, is every sky that can be seen from Earth: every location at every time of year given every position of the Sun, Moon, and planets. And SDSS, for all its millions of observations, has only mapped a fraction of a fraction of the objects in 35% of that sky. The remaining sky holds light we cannot see.
Our solar system’s home on the Orion Arm of the Milky Way puts us in an entirely average position within the universe. We orbit the galaxy’s core much like our own planet orbits our sun. We sit near an edge, shielded from the violent activity of the supermassive black hole in the galactic center. Although our galaxy is relatively flat, the thick clouds of gas and distribution of stars within the Milky Way prevent us from seeing the expanse of space on the other side, leaving us with only narrow angles of observation—looking out from the broad sides, with our view illustrated as > / < (in which / is the entire galaxy)—from which to understand the universe.
Within the 35% of sky visible from our galactic position, further factors complicate our view. The universe is expanding, in some regions faster than the speed of light; light from these galaxies will never reach Earth. We are also limited, of course, by time: we can only see light that has had time, in the 13.8 billion years of the universe’s existence, to reach us. What we see in the observable universe may be only a fraction of the entire universe, about which we can draw few conclusions. Is it finite or infinite? Bounded or unbounded? What is the extent of the light we cannot see?
We are also constrained by our capacity. The entire history of the universe is happening all around us at all times. We cannot look everywhere at once. Sometimes, however, we get lucky. In 2009, a combination of space and land telescopes in the United States, Italy, and Chile observed gamma-ray burst GRB 090423 over the course of 3.5 days. At the time, it was the oldest astronomical event ever directly observed: the light from the burst took more than 13 billion years to reach Earth, and within 4 days of the initial burst, it was no longer detectable. If our technology had been less advanced, for example, or our planet had been on the other side of the sun, we would not have witnessed this tiny breath of cosmological history.
The brilliance of the Milky Way, a stunning expanse from locations with sufficiently dark skies, obstructs our view of much of the universe. Yet even that remarkable view is only visible from sites distant from light we create on Earth—the luminescence of streetlights, security lights, and other sky-lightening markers of modern life. The Moon, too, creates a lustrous interruption in our night sky, its brilliance causing finer points of distant light to fade from resolution. Upon the rising sun, the cosmos disappears; we are left alone in a characterless, blue expanse.
When everything is illuminated, we see nothing beyond ourselves.