My most vivid encounter with the vastness of nature occurred years ago on the Aegean Sea. My wife and I had chartered a sailboat for a two-week holiday in the Greek islands. After setting out from Piraeus, we headed south and hugged the coast, which we held three or four miles to our port. In the thick summer air, the distant shore appeared as a hazy beige ribbon—not entirely solid, but a reassuring line of reference. With binoculars, we could just make out the glinting of houses, fragments of buildings.

Then we passed the tip of Cape Sounion and turned west toward Hydra. Within a couple of hours, both the land and all other boats had disappeared. Looking around in a full circle, all we could see was water, extending out and out in all directions until it joined with the sky. I felt insignificant, misplaced, a tiny odd trinket in a cavern of ocean and air.

Naturalists, biologists, philosophers, painters, and poets have labored to express the qualities of this strange world that we find ourselves in. Some things are prickly, others are smooth. Some are round, some jagged. Luminescent or dim. Mauve colored. Pitter-patter in rhythm. Of all these aspects of things, none seems more immediate or vital than size. Large versus small. Consciously and unconsciously, we measure our physical size against the dimensions of other people, against animals, trees, oceans, mountains. As brainy as we think ourselves to be, our bodily size, our bigness, our simple volume and bulk are what we first present to the world. Somewhere in our fathoming of the cosmos, we must keep a mental inventory of plain size and scale, going from atoms to microbes to humans to oceans to planets to stars. And some of the most impressive additions to that inventory have occurred at the high end. Simply put, the cosmos has gotten larger and larger. At each new level of distance and scale, we have had to contend with a different conception of the world that we live in.

The prize for exploring the greatest distance in space goes to a man named Garth Illingworth, who works in a ten-by-fifteen-foot office at the University of California, Santa Cruz. Illingworth studies galaxies so distant that their light has traveled through space for more than 13 billion years to get here. His office is packed with tables and chairs, bookshelves, computers, scattered papers, issues of Nature, and a small refrigerator and a microwave to fuel research that can extend into the wee hours of the morning.

Like most professional astronomers these days, Illingworth does not look directly through a telescope. He gets his images by remote control—in his case, quite remote. He uses the Hubble Space Telescope, which orbits Earth once every ninety-seven minutes, high above the distorting effects of Earth’s atmosphere. Hubble takes digital photographs of galaxies and sends the images to other orbiting satellites, which relay them to a network of earthbound antennae; these, in turn, pass the signals on to the Goddard Space Flight Center in Greenbelt, Maryland. From there the data is uploaded to a secure website that Illingworth can access from a computer in his office.

The most distant galaxy Illingworth has seen so far goes by the name UDFj-39546284 and was documented in early 2011. This galaxy is about 100,000,000,000,000,000,000,000 miles away from Earth, give or take. It appears as a faint red blob against the speckled night of the distant universe—red because the light has been stretched to longer and longer wavelengths as the galaxy has made its lonely journey through space for billions of years. The actual color of the galaxy is blue, the color of young, hot stars, and it is twenty times smaller than our galaxy, the Milky Way. UDFj-39546284 was one of the first galaxies to form in the universe.

“That little red dot is hellishly far away,” Illingworth told me recently. At sixty-five, he is a friendly bear of a man, with a ruddy complexion, thick strawberry-blond hair, wire-rimmed glasses, and a broad smile. “I sometimes think to myself: What would it be like to be out there, looking around?”

One measure of the progress of human civilization is the increasing scale of our maps. A clay tablet dating from about the twenty-fifth century b.c. found near what is now the Iraqi city of Kirkuk depicts a river valley with a plot of land labeled as being 354 iku (about thirty acres) in size. In the earliest recorded cosmologies, such as the Babylonian Enuma Elish, from around 1500 b.c., the oceans, the continents, and the heavens were considered finite, but there were no scientific estimates of their dimensions. The early Greeks, including Homer, viewed Earth as a circular plane with the ocean enveloping it and Greece at the center, but there was no understanding of scale. In the early sixth century b.c., the Greek philosopher Anaximander, whom historians consider the first mapmaker, and his student Anaximenes proposed that the stars were attached to a giant crystalline sphere. But again there was no estimate of its size.

The first large object ever accurately measured was Earth, accomplished in the third century b.c. by Eratosthenes, a geographer who ran the Library of Alexandria. From travelers, Eratosthenes had heard the intriguing report that at noon on the summer solstice, in the town of Syene, due south of Alexandria, the sun casts no shadow at the bottom of a deep well. Evidently the sun is directly overhead at that time and place. (Before the invention of the clock, noon could be defined at each place as the moment when the sun was highest in the sky, whether that was exactly vertical or not.) Eratosthenes knew that the sun was not overhead at noon in Alexandria. In fact, it was tipped 7.2 degrees from the vertical, or about one fiftieth of a circle—a fact he could determine by measuring the length of the shadow cast by a stick planted in the ground. That the sun could be directly overhead in one place and not another was due to the curvature of Earth. Eratosthenes reasoned that if he knew the distance from Alexandria to Syene, the full circumference of the planet must be about fifty times that distance. Traders passing through Alexandria told him that camels could make the trip to Syene in about fifty days, and it was known that a camel could cover one hundred stadia (almost eleven and a half miles) in a day. So the ancient geographer estimated that Syene and Alexandria were about 570 miles apart. Consequently, the complete circumference of Earth he figured to be about 50 × 570 miles, or 28,500 miles. This number was within 15 percent of the modern measurement, amazingly accurate considering the imprecision of using camels as odometers.

As ingenious as they were, the ancient Greeks were not able to calculate the size of our solar system. That discovery had to wait for the invention of the telescope, nearly two thousand years later. In 1672, the French astronomer Jean Richer determined the distance from Earth to Mars by measuring how much the position of the latter shifted against the background of stars from two different observation points on Earth. The two points were Paris (of course) and Cayenne, French Guiana. Using the distance to Mars, astronomers were also able to compute the distance from Earth to the sun, approximately 100 million miles.

A few years later, Isaac Newton managed to estimate the distance to the nearest stars. (Only someone as accomplished as Newton could have been the first to perform such a calculation and have it go almost unnoticed among his other achievements.) If one assumes that the stars are similar objects to our sun, equal in intrinsic luminosity, Newton asked, how far away would our sun have to be in order to appear as faint as nearby stars? Writing his computations in a spidery script, with a quill dipped in the ink of oak galls, Newton correctly concluded that the nearest stars are about 100,000 times the distance from Earth to the sun, about 10 trillion miles away. Newton’s calculation is contained in a short section of his Principia titled simply “On the distance of the stars.”

Newton’s estimate of the distance to nearby stars was larger than any distance imagined before in human history. Even today, nothing in our experience allows us to relate to it. The fastest most of us have traveled is about 500 miles per hour, the cruising speed of a jet. If we set out for the nearest star beyond our solar system at that speed, it would take us about 5 million years to reach our destination. If we traveled in the fastest rocket ship ever manufactured on Earth, the trip would last 100,000 years, at least a thousand human life spans.

But even the distance to the nearest star is dwarfed by the measurements made in the early twentieth century by Henrietta Leavitt, an astronomer at the Harvard College Observatory. In 1912, she devised a new method for determining the distances to faraway stars. Certain stars, called Cepheid variables, were known to oscillate in brightness. Leavitt discovered that the cycle times of such stars are closely related to their intrinsic luminosities. More luminous stars have longer cycles. Measure the cycle time of such a star and you know its intrinsic luminosity. Then, by comparing its intrinsic luminosity with how bright it appears in the sky, you can infer its distance, just as you could gauge the distance to an approaching car at night if you knew the wattage of its headlights. Cepheid variables are scattered throughout the cosmos. They serve as cosmic distance signs in the highway of space.

Using Leavitt’s method, astronomers were able to determine the size of the Milky Way, a giant congregation of about 200 billion stars. To express such mind-boggling sizes and distances, twentieth-century astronomers adopted a new unit called the light-year, the distance that light travels in a year—about 6 trillion miles. The nearest stars are several light-years away. The diameter of the Milky Way has been measured at about 100,000 light-years. In other words, it takes a ray of light 100,000 years to travel from one side of the Milky Way to the other.

There are galaxies beyond our own. They have names like Andromeda (one of the nearest), Sculptor, Messier 87, Malin 1, IC 1101. The average distance between galaxies, again determined by Leavitt’s method, is about twenty galactic diameters, or 2 million light-years. To a giant cosmic being leisurely strolling through the universe and not limited by distance or time, galaxies would appear as illuminated mansions scattered about the dark countryside of space. As far as we know, galaxies are the largest objects in the cosmos. If we sorted the long inventory of material objects in nature by size, we would start with subatomic particles like electrons and end up with galaxies.

Over the past century, astronomers have been able to probe deeper and deeper into space, looking out to distances of hundreds of millions of light-years and farther. A question naturally arises: Could the physical universe be unending in size? That is, as we build bigger and bigger telescopes sensitive to fainter and fainter light, will we continue to see objects farther and farther away—like the third emperor of the Ming Dynasty, Yongle, who surveyed his new palace in the Forbidden City and walked from room to room to room, never reaching the end?

Here we must take into account a curious relationship between distance and time. Because light travels at a fast (186,000 miles per second) but not infinite speed, when we look at a distant object in space we must remember that a significant amount of time has passed between the emission of the light and the reception at our end. The image we see is what the object looked like when it emitted that light. If we look at an object 186,000 miles away, we see it as it appeared one second earlier; at 1,860,000 miles away, we see it as it appeared ten seconds earlier; and so on. For extremely distant objects, we see them as they were millions or billions of years in the past.

Now the second curiosity. Since the late 1920s we have known that the universe is expanding, and that as it does so it is thinning out and cooling. By measuring the current rate of expansion, we can make good estimates of the moment in the past when the expansion began—the Big Bang—which was about 13.7 billion years ago, a time when no planets or stars or galaxies existed and the entire universe consisted of a fantastically dense nugget of pure energy. No matter how big our telescopes, we cannot see beyond the distance light has traveled since the Big Bang. Farther than that, and there simply hasn’t been enough time since the birth of the universe for light to get from there to here. This giant sphere, the maximum distance we can see, is only the observable universe. But the universe could extend far beyond that.

In his office in Santa Cruz, Garth Illingworth and his colleagues have mapped out and measured the cosmos to the edge of the observable universe. They have reached out almost as far as the laws of physics allow. All that exists in the knowable universe—oceans and sky; planets and stars; pulsars, quasars, and dark matter; distant galaxies and clusters of galaxies; and great clouds of star-forming gas—has been gathered within the cosmic sensorium gauged and observed by human beings.

“Every once in a while,” says Illingworth, “I think: By God, we are studying things that we can never physically touch. We sit on this miserable little planet in a midsize galaxy and we can characterize most of the universe. It is astonishing to me, the immensity of the situation, and how to relate to it in terms we can understand.”

The idea of Mother Nature has been represented in every culture on Earth. But to what extent is the new universe, vastly larger than anything conceived of in the past, part of nature? One wonders how connected Illingworth feels to this astoundingly large cosmic terrain, to the galaxies and stars so distant that their images have taken billions of years to reach our eyes. Are the little red dots on his maps part of the same landscape that Wordsworth and Thoreau described, part of the same environment of mountains and trees, part of the same cycle of birth and death that orders our lives, part of our physical and emotional conception of the world we live in? Or are such things instead digitized abstractions, silent and untouchable, akin to us only in their (hypothesized) makeup of atoms and molecules? And to what extent are we human beings, living on a small planet orbiting one star among billions of stars, part of that same nature?

The heavenly bodies were once considered divine, made of entirely different stuff than objects on Earth. Aristotle argued that all matter was constituted from four elements: earth, fire, water, and air. A fifth element, ether, he reserved for the heavenly bodies, which he considered immortal, perfect, and indestructible. It wasn’t until the birth of modern science, in the seventeenth century, that we began to understand the similarity of heaven and Earth. In 1610, using his new telescope, Galileo noted that the sun had dark patches and blemishes, suggesting that the heavenly bodies are not perfect. In 1687, Newton proposed a universal law of gravity that would apply equally to the fall of an apple from a tree and to the orbits of planets around the sun. Newton then went further, suggesting that all the laws of nature apply to phenomena in the heavens as well as on Earth. In later centuries, scientists used our understanding of terrestrial chemistry and physics to estimate how long the sun could continue shining before depleting its resources of energy; to determine the chemical composition of stars; to map out the formation of galaxies.

Yet even after Galileo and Newton, there remained another question: Were living things somehow different from rocks and water and stars? Did animate and inanimate matter differ in some fundamental way? The “vitalists” claimed that animate matter had some special essence, an intangible spirit or soul, while the “mechanists” argued that living things were elaborate machines and obeyed precisely the same laws of physics and chemistry as did inanimate material. In the late nineteenth century, two German physiologists, Adolf Eugen Fick and Max Rubner, each began testing the mechanistic hypothesis by painstakingly tabulating the energies required for muscle contraction, body heat, and other physical activities and comparing these energies against the chemical energy stored in food. Each gram of fat, carbohydrate, and protein had its energy equivalent. Rubner concluded that the amount of energy used by a living creature was exactly equal to the energy it consumed in its food. Living things were to be viewed as complex arrangements of biological pulleys and levers, electric currents, and chemical impulses. Our bodies are made of the same atoms and molecules as stones, water, and air.

And yet many had a lingering feeling that human beings were somehow separate from the rest of nature. Such a view is nowhere better illustrated than in the painting Tallulah Falls (1841), by George Cooke, an artist associated with the Hudson River School. Although this group of painters celebrated nature, they also believed that human beings were set apart from the natural world. Cooke’s painting depicts tiny human figures standing on a small promontory above a deep canyon. The people are dwarfed by tree-covered mountains, massive rocky ledges, and a waterfall pouring down to the canyon below. Not only insignificant in size compared with their surroundings, the human beings are mere witnesses to a scene they are not part of and never could be. Just a few years earlier, Ralph Waldo Emerson had published his famous essay “Nature,” an appreciation of the natural world that nonetheless held humans separate from nature, at the very least in the moral and spiritual domain: “Man is fallen; nature is erect.”

Today, with various back-to-nature movements attempting to resist the dislocations brought about by modernity, and with our awareness of Earth’s precarious environmental state ever increasing, many people feel a new sympathy with the natural world on this planet. But the gargantuan cosmos beyond remains remote. We might understand at some level that those tiny points of light in the night sky are similar to our sun, made of atoms identical to those in our bodies, and that the cavern of outer space extends from our galaxy of stars to other galaxies of stars, to distances that would take light billions of years to traverse. We might understand these discoveries in intellectual terms, but they are baffling abstractions, even disturbing, like the notion that each of us once was the size of a dot, without mind or thought. Science has vastly expanded the scale of our cosmos, but our emotional reality is still limited by what we can touch with our bodies in the time span of our lives. George Berkeley, the eighteenth-century Irish philosopher, argued that the entire cosmos is a construct of our minds, that there is no material reality outside our thoughts. As a scientist, I cannot accept that belief. At the emotional and psychological level, however, I can have some sympathy with Berkeley’s views. Modern science has revealed a world as far removed from our bodies as colors are from the blind.

Very recent scientific findings have added yet another dimension to the question of our place in the cosmos. For the first time in the history of science, we are able to make plausible estimates of the rate of occurrence of life in the universe. In March 2009, NASA launched a spacecraft called Kepler whose mission was to search for planets orbiting in the “habitable zone” of other stars. The habitable zone is the region in which a planet’s surface temperature is not so cold as to freeze water and not so hot as to boil it. For many reasons, biologists and chemists believe that liquid water is required for the emergence of life, even if that life may be very different from life on Earth. Dozens of candidates for such planets have been found, and we can make a rough preliminary calculation that something like 3 percent of all stars are accompanied by a potentially life-sustaining planet. The totality of living matter on Earth—humans and animals, plants, bacteria, and pond scum—makes up 0.00000001 percent of the mass of the planet. Combining this figure with the results from the Kepler mission, and assuming that all potentially life-sustaining planets do indeed have life, we can estimate that the fraction of stuff in the visible universe that exists in living form is something like 0.000000000000001 percent, or one millionth of one billionth of 1 percent. If some cosmic intelligence created the universe, life would seem to have been only an afterthought. And if life emerges by random processes, vast amounts of lifeless material are needed for each particle of life. Such numbers cannot help but bear upon the question of our significance in the universe.

Decades ago, when I was sailing with my wife in the Aegean Sea, in the midst of unending water and sky, I had a slight inkling of infinity. It was a sensation I had not experienced before, accompanied by feelings of awe, fear, sublimity, disorientation, alienation, and disbelief. I set a course for 255°, trusting in my compass—a tiny disk of painted numbers with a sliver of rotating metal—and hoped for the best. In a few hours, as if by magic, a pale ocher smidgen of land appeared dead ahead, a thing that drew closer and closer, a place with houses and beds and other human beings.