As a kid visiting the Oregon coast I often wondered, “How wide is the ocean, and what is there beyond the horizon?” As I grew older and turned my sights to the night sky, I wondered something very similar: “How far away are the stars, and are there other planets there?” Even though very few of us have ever circumnavigated the globe, and no human being has ever ventured into space beyond the Moon, we do know some of the answers to these questions. Immensity isn’t immeasurable. While these vast numbers may make little sense in our daily lives, we at least know they are known.
Consider what it must have been like to live in a world where this was not true: where the sense of immeasurability, the certainty of the unfathomable, was commonplace, and the thought that the world could be known was a novel idea. The philosopher Anaxagoras was born in about 500 BCE in the eastern Mediterranean on what is now the coast of Turkey. It was a time when philosophy had only recently turned its attention to the natural world. Less than a hundred years before, Thales of Miletus supposedly predicted the solar eclipse that ended a war, thus implying that our world was predictable and events were not just the random whims of the gods.
In such a world of physical phenomena, Anaxagoras was the first, as far as we know, to understand that eclipses occur when one heavenly body blocks the light from another. This rejection of gods and dragons as the causes of eclipses was a revolutionary thought by itself, but Anaxagoras took it further: If solar eclipses happened only because the Earth had moved into the shadow of the Moon, he reasoned, then the size of the shadow must tell us something about the size of the Moon. Additionally, since the Moon covered the Sun, the Sun must be farther away. Yet to appear nearly the same size, the Sun must be larger than the Moon. Herein lies the power of scientific thought: measure the extent of the shadow sweeping across the Earth, and you know the Moon must be at least as big as the shadow, and the Sun larger still. Mysticism provided no such opportunity: if eclipses occur when a demon devours the Sun, there is no reason to believe that any measurement we make here on Earth should reveal the demon’s size.
On February 17, 478 BCE, the shadow of an annular eclipse spread across the Mediterranean Sea and crossed the Greek islands and peninsula of the Peloponnese, creating a “ring of fire” in the sky that was visible for almost six minutes. Anaxagoras, living in Athens, would have been living along the midline of annularity and surely would have seen the sight, but he could not, all by himself, in only six minutes, measure the size of the shadow across the countryside. And yet in a stroke of genius, he found the answer to his question: he simply went down to the seashore and asked arriving sailors what they had seen. At that time, Athens was the center of trade for ships from all over the eastern Mediterranean. If sailors at sea had seen a ring of fire in the sky, they would remember where they had been when they had seen it. The locations of all those who did and did not see the spectacle revealed the extent of the shadow across the surrounding seascape. Without going farther than the local seaport, Anaxagoras measured the Moon.
While we do not have Anaxagoras’s own words as to what he concluded, we do have the writings of those who came after. Five hundred years later, the Roman historian Plutarch wrote, “Anaxagoras [says that the Moon] is as large as the Peloponnesus.” Hippolytus of Rome, a third-century father of the Christian church, wrote in his Refutation of All Heresies that, according to Anaxagoras, “the sun exceeds the Peloponnesus in size.” The story of Anaxagoras standing on the beach measuring the size of the Moon is the story of astronomy. We are a species confined to our own world (or at best, our own solar system). Yet from this one vantage spot we have had to survey the universe on whose shores we stand. To do so we have had to study eclipses, transits (when small things move in front of big things), and occultations (when big things move in front of small things). Astronomy is made possible, in part, by the shadows that span the stars.
Standing on the celestial seashore, let’s pace out our universe, starting from the world we see by day to the stars we see at night. At each step we will learn where we are and how far we’ve come. What is the simplest way to measure distance? We can walk. We measure distance in feet (at least in the United States), and it’s no accident that a foot is about the size of our own feet. How far can a person measure precisely by walking? In the Mediterranean of the third century BCE, bematists were men who could walk at a precise and constant pace, and they were paid to do so. You could hire such a man to accurately measure long distances across the landscape. Bematists were used along the Egyptian Nile, which during its annual floods erased the features marking the boundaries between fields. Bematists were particularly suited to pace off the long, flat, featureless landscape along the Nile south from Alexandria to Syene, which they found to be 5,000 stadia apart (around 520 modern miles, depending on the exact definition of stadia used). We know this distance because sometime in around 240 BCE, Eratosthenes of Cyrene, the chief librarian of Alexandria, used it to find the size of the world.
Eratosthenes had heard that on the summer solstice, the noonday Sun would shine straight down a well in Syene and cast no shadow. He knew no such thing happened in Alexandria on that or any other day, so one of two possibilities must be true: either the Earth was flat and the Sun was very close (much like a cloud that hangs over one town appears far to the south as seen from another), or the Sun was far away and the Earth was round. The answer could be found by looking at the Moon during a lunar eclipse. Aristotle had already noted a century before Eratosthenes’s experiment that during every lunar eclipse the Earth’s shadow looked like a circle. No matter where the eclipse occurred in the sky, the shadow the Earth cast never changed. The only figure that looked the same from any direction was a sphere.
Since the Earth’s shadow already confirmed the Earth was round, the only explanation for the different lengths of the Sun’s shadows in Syene and Alexandria was the curvature of the Earth. From the difference in shadows and the distance between them, the circumference of the Earth was revealed.
Here’s how the principle works: Imagine that one day in the Hawaiian Islands, you notice your own shadow directly at your feet, while flag poles and Tiki totems cast no shadows at all. You look overhead and there is the Sun at the zenith, the highest point in the sky. Hawaiians call it Lahaina Noon after the name of a town on the island of Maui where this happens twice each year. You immediately call your friend in Puerto Rico, who is unimpressed. Right at that instant, she is watching a spectacular sunset, with the Sun touching the waters of the Caribbean on the horizon. At that moment, the two of you see the Sun 90 degrees apart in the sky, that is, exactly one-quarter of a circle apart (90o/360o = ¼). You must therefore be a quarter of a circle, a quarter of the Earth’s circumference, away from one another. Measure the distance between you, multiply by four, and you know the full distance around the Earth.
This is precisely what Eratosthenes did. At the moment that the Sun was directly overhead and shadows disappeared in Syene, he measured the lengths of shadows in Alexandria and concluded that there was a change in the Sun’s position of 7.2 degrees. This difference meant that the two cities were 1/50th (7.2o/360o) of the way around the Earth from one another. Since the distance on foot between them was 5,000 stadia, the entire Earth, Eratosthenes reasoned, must be 250,000 stadia around. Depending on the precise length of a stadia, his value for the Earth’s circumference may have been off by as little as 2 percent from the actual value we now know. More important than this precision, however, is the very idea that it could be done.
For Eratosthenes’s method to work, it was vital that two distant observers be looking at the same thing (the Sun, in this case) at exactly the same time and see it forming different shadows in different places. But how can we be sure that any two people on Earth are looking at the same thing in the sky at exactly the same moment? Simple: use a lunar eclipse.
As early as 150 BCE, the astronomer Hipparchus suggested using lunar eclipses to determine longitude (locations east and west) around the Mediterranean. To do this accurately, however, requires paying attention to time. When the Moon moves into the shadow of the Earth and is eclipsed, everyone on the night-side of Earth sees it happen at the same instant. However, the local time at which each person sees the eclipse occur depends on the person’s location east–west across the nighttime Earth. If we know the difference in local time for an event everyone experiences at once, we can tell how far east and west they must be from one another. Since it takes the Earth twenty-four hours to turn 360 degrees, the Earth must turn 15 degrees per hour. For every difference of one hour in local time, two observers must therefore be separated by 15 degrees around the Earth. If I see the lunar eclipse begin at 10:00 in the evening, while my friend sees it start at 1:00 in the morning, the difference in local time is three hours, and we must be 45 degrees around the world from one another.
Once Eratosthenes measured how many stadia there were per degree, then a difference in local time between two observers became a difference in stadia around the planet. For this reason, it would be useful to have a tool that could calculate the expected dates and times of eclipses in a known location (against which to compare the local time at which the eclipses were seen while traveling). Just such an eclipse calculator was found in 1901 in the remains of a shipwreck off the Greek island of Antikythera. What is now called the Antikythera Device is an intricate mechanism of bronze gears that have been shown to calculate various astronomical quantities, including the position of the Sun against the zodiac (and thus the date), the phase of the Moon, and the date and time of solar and lunar eclipses. To calculate eclipses, its dials appear to have tracked the Saros and Exeligmos cycles. From matching the intervals of known eclipses in the ancient world with the intervals predicted by the markings on the dials, scholars have dated the device to sometime around the third century BCE. That’s 1,500 years before metal gears would once more be used in Europe to make clocks for telling time and modeling the heavens.
No one knows if there were other copies of the Antikythera Device traveling the Mediterranean onboard other ships, but we do know that in the centuries that followed, navigators set sail with books and scrolls filled with detailed astronomical tables filled with the positions of Sun, Moon, and stars and the times of their eclipses. Christopher Columbus was just such a navigator.
Thanks to Washington Irving’s 1828 biography of Columbus, most Americans learn that it was Columbus alone who believed the world was round and that the East could be reached by sailing west. In truth, it wasn’t Columbus’s beliefs about the shape of the Earth, but rather about the size of the Earth, that set him apart from his contemporaries.
By a combination of wishful thinking, selective observations, and a confusion of Roman for Arabic miles, Columbus believed the Earth was a quarter smaller than Eratosthenes had found. He also thought the distance from Europe to Asia was so great—greater than anyone else believed—that the open sea between the Canary Islands (off the coast of Africa) and Japan in the East was no more than 2,400 nautical miles (only one-sixth of the way around the world). The actual distance along the route he had planned is closer to 10,600 nautical miles, over halfway around and pretty close to what the best minds of the day had calculated.
It’s no wonder everyone else thought he was mad. By all rights, Columbus should have been sailing out into an ocean spanning over half the planet; it was only by luck that he found the Caribbean approximately where he expected to find the eastern reaches of Asia. It must have been so confusing when nothing he found was as he expected.
When, on his fourth voyage, Columbus used a lunar eclipse to scare the local populace, the difference in time between his hourglass and what his almanac predicted for observers back in Europe, should have revealed the mistake of where he actually was. Columbus thought he was somewhere off the coast of China, 107 degrees of longitude from Spain across the sea. Since the world turns 15 degrees per hour, there should be about 7 hours and 15 minutes of difference between his local time and clocks back in Spain. In reality, Columbus was only about 71 degrees west of Spain, and so the difference in time would have been about 4 hours and 44 minutes.
Through a lucky combination of error in reading his almanac and the difficulties of keeping accurate local time using sand in an hourglass, the eclipsed Moon wound up rising over the eastern ocean almost exactly when he expected. It was a complete and utter coincidence. Had he really been 7 hours west of Spain, Columbus would have discovered Arizona. Columbus would go to his grave never knowing how lost he actually had been. Not until Sir Walter Raleigh observed a lunar eclipse from Roanoke Island off the coast of Virginia in 1584 was the width of the Atlantic Ocean actually known.
Over the next three hundred years, explorers continued to map North America using the sky. President Thomas Jefferson had captains Meriwether Lewis and William Clark trained in astronomy to find their way across the continent. On the night of January 14, 1805, in the village of the Mandan tribe in modern North Dakota, Lewis wrote in his journal: “Observed an Eclips [sic] of the Moon. I had no other glass to assist me in this observation but a small refracting telescope belonging to my sextant, which however was of considerable service, as it enabled me to define the edge of the moon’s immage [sic] with much more precision than I could have done with the natural eye.” Using his astronomical almanacs, Lewis calculated his correct longitude to within 85 miles.
Solar eclipses are even better tools for finding one’s location (owing to the dramatically instant onset of totality). The logs of eighteenth- and nineteenth-century solar eclipse expeditions are full of detailed and rather tedious tables of times and positions on the sky of eclipses against which the towns, rivers, and mountains of continent-wide countries and globe-spanning empires were measured. In 1869, Major John Wesley Powell and the men under his command became the first Europeans (and possibly first people ever) to navigate the Colorado River by boat and to map the last remaining blank spots in America. Powell’s almanac predicted a solar eclipse on their journey, but the day of the eclipse found the one-armed Civil War veteran at the bottom of the 2,000-foot-deep Grand Canyon, out of view of the Sun. George Young Bradley, a boatman and geologist on the expedition, wrote in his private journal that “Major & brother have climbed the mountain to observe the eclipse but think it almost or quite a total failure for it has rained almost or quite all the P.M. We could see the sun from camp when it was about half covered but it clouded immediately and before the cloud passed it was behind the bluffs. Major has not come in. Cannot tell whether he saw it or not. If he did we shall have our Longitude.”
Due to the typical summer monsoon clouds of the canyon country, it didn’t clear, and so his exact position would remain a mystery. If only the Earth had more moons, making for more frequent eclipses. Sadly, the Earth doesn’t; but Jupiter does.
In January 1610, Galileo discovered four large moons around Jupiter, each one about the same size as our own Moon. The closest of these Galilean satellites, Io, orbits Jupiter once every 1.8 days, and each time around it is eclipsed, or, more accurately, occulted, by Jupiter’s disk. Galileo himself realized that these occultations happened with such regularity that they could be used to find longitude on Earth. Construct a table of eclipse times as visible from Paris, for instance, and Jupiter turns into a tiny clock in the sky. No matter where you may be, observe a Jovian eclipse through a telescope, and you immediately know the time back in Paris to compare with your own.
When King Louis XIV sought to make France the world leader of science in the late 1600s, he employed astronomers to use Jupiter’s moons to make the world’s most precise map of France. His astronomers traveled all over the country with their telescopes, recording their Jovian eclipses and calculating their distance from Paris. It was the most accurate map ever produced up to that time, and it revealed that many roads and distances were actually shorter than had been believed. In her history of longitude, Dava Sobel wrote that upon seeing the new map, the king “complained that he was losing more territory to his astronomers than to his enemies.”
In 1793, Alexander MacKenzie and his team of ten men lugged a telescope across the breadth of what would become Canada (a decade before Lewis and Clark would do the same in the United States). They found their location along the Pacific Coast by seeing the tiny eclipse of Jupiter’s largest moon, Ganymede. Over a decade later, as Lewis and Clark were on their way back from the Pacific, President Jefferson commanded Lieutenant Zebulon Pike to determine the position of the Rocky Mountains in Colorado by means of Jupiter’s moons, even though the Rockies were still under Spanish rule. Spanish troops captured Pike and his men in 1806, and before letting them go, confiscated all their astronomical observations. Pike’s Peak in Colorado is named in his honor; fittingly, anyone now can drive to its top with a GPS unit triangulating the car’s position from satellites in space. More than two hundred years after Pike’s expedition, we still know where we are and where we are going by means of astronomy. These methods of using triangulation and eclipses are not, however, limited to distances here on Earth. The same techniques Anaxagoras used so long ago reveal that the cosmos is much larger than anyone had ever believed. To understand how big, let us turn our eyes back to the shadow our planet casts and look again at a lunar eclipse.
About two hundred years after Anaxagoras wrote that the Moon was larger than the peninsula and islands of the Peloponnese, another Greek philosopher, Aristarchus, realized that when the Moon entered the Earth’s shadow, we instantly saw its size in comparison to our own. Aristarchus measured the curve of the umbra (the darkest part of the Earth’s shadow) as it crossed the face of the Moon and concluded that between 2.5 and 3 Moons would fit side by side across our shadow. When at last Eratosthenes measured the size of the Earth, the size of the Moon was suddenly known. But again, Aristarchus went further. In his On the Sizes and Distances of the Sun and the Moon, written in around 280 BCE, he showed that if you knew the actual size of the Moon, then you could determine how far away it must be to appear as small as it does in the sky. We do this today with high school trigonometry.
As for the distance to the Sun, Aristarchus imagined a triangle formed between the Earth, Sun, and Moon when exactly half the Moon was lit by the Sun as viewed from Earth (and so a 90o angle existed at the corner with the Moon). Measure the angle at the Earth (the angle between Moon and Sun on the sky), and if you know the distance to the Moon, then, from trigonometry, you know the distance to the Sun. Sadly, the distances were so vast, and the angles so close to 90 degrees (and his ability to measure positions on the sky so imprecise), that there was no way his method would work in actual practice.
In the end, Aristarchus’s calculations pointed to a Sun twenty times farther away than the Moon and consequently twenty times larger (since both appear the same size in the sky). The reality was different, off by an additional factor of twenty, but the fact that it revealed a Sun very much larger than the Earth was profound. If the Sun was so much larger than the Earth, then why would the Sun orbit the Earth and not the other way around? It was an early argument for those who believed in a heliocentric universe.
FIGURE 3.1. When the Moon is at first quarter, the Sun, Earth, and Moon form a right triangle, and trigonometry says that if you know two angles and the distance between them, the distance to the other point is uniquely known. (Image by the author)
One proof that geocentric proponents gave for why this was impossible stated that if the Earth really did move through space, then the constellations should change size and shape with the seasons as we orbited the Sun. We would draw close to some stars and move away from others. Yet no one had ever seen this happen. But how much the stars should change their positions depended on two things: the distance to the nearest stars (whose position should change the most compared with those that were farther away); and our distance from the Sun—how far we actually moved through space as we traveled from one side of the Sun to the other (January to July, for instance).
But no one knew how far away the Sun really was. In fact, everything we know about the size of the solar system, including even the actual size of the planets, depends on knowing their distance from us. As of the nineteenth century, all of these distances were simply known in increments of the distance between Earth and Sun. For this reason, this distance was called the Astronomical Unit (AU), and its calculation was one of the great questions of astronomy in the centuries after Galileo.
FIGURE 3.2. As the Earth orbits the Sun, a nearby star will appear to shift its position relative to more distant stars. (Image by the author)
The British astronomer Sir Edmond Halley was the first to suggest a means to solve this problem using a phenomenon called parallax on the planet Venus. On those rare occasions when Venus passes directly between the Earth and the Sun (creating a tiny eclipse, or a transit), different observers at different locations on Earth see Venus take slightly different paths across the solar disk. For observers at opposite ends of the Earth, the greater the difference in apparent paths, the closer Venus had to be. You can see this for yourself by holding your thumb a few inches in front of your face. Look at it with first one eye and then the other: your thumb shifts against the background wall depending on which eye you use and how far away you hold your thumb. The closer you hold it, the larger the shift. Our brains are hardwired to interpret the different sized shifts as differences in distance, and thus we have stereo vision that renders our world in 3-D.
Halley proposed that astronomers travel as far as possible away from each other across the globe and each record the path of Venus’s transit across the Sun. Afterward, a comparison of the observations from the far-flung observers, along with simple trigonometry, would help them determine the distance to Venus. Since Kepler’s third law provided the distance between all the planets in terms of Astronomical Units, once one distance was measured, all the rest would become known. Because Venus orbits the Sun once every 225 days, in principle that means that it should lap us in our race around the Sun at least once every year. But like the Moon’s orbit around the Earth, Venus’s orbit around the Sun is also tilted. This geometry renders those moments rare when all three bodies align. Transits of Venus come in pairs, separated by eight years. Unfortunately, over a hundred years go by between transit pairs, meaning that generations of astronomers can live and die without ever seeing one such event. Halley concluded that the next alignment would not be visible until 1761 (followed eight years later by another)—long after he would be dead. Writing in Latin so as to reach as wide an audience of scientists as possible, Halley wrote in 1716:
I would have several observations made of the same phenomenon in different parts [of the world], both for further confirmation, and lest a single observer should happen to be disappointed by the intervention of clouds from seeing what I know not if those either of the present or following age shall ever see again; and upon which, the certain and adequate solution of the noblest, and otherwise most difficult problem depends. Therefore again and again, I recommend it to the curious strenuously to apply themselves to this observation.
All over Europe, scientific societies answered his call, and as each transit opportunity approached, they launched expeditions that sailed the world for astronomy. One of the most successful transit expeditions was that of Captain James Cook, who in 1768 set sail on his first voyage into the Pacific. His destination was the island of Tahiti, where he and his crew established a small observatory at a place still known as Point Venus. He wrote: “Saturday, 3rd [June 1769]. This day proved as favourable to our purpose as we could wish. Not a Cloud was to be seen the whole day, and the Air was perfectly Clear, so that we had every advantage we could desire in observing the whole of the Passage of the planet Venus over the Sun’s Disk.”
On this voyage Cook would go on to explore New Zealand and the east coast of Australia, where his botanist, Sir Joseph Banks, discovered a multitude of plants and animals formerly unknown to Western science. This voyage and the two that came after made Captain Cook a hero in England and the subject of local history studies for schoolchildren, including myself, all over the Pacific Ocean and on multiple continents.
At the other end of the spectrum, and other end of the world, one of the most unsuccessful transit expeditions was surely that of the French astronomer Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil, who at the behest of the French Academy of Sciences set out for the Indian Ocean to observe the transit of 1761. Le Gentil’s destination was Pondicherry, India, a site specifically suggested by Halley himself. When he arrived in the Indian Ocean, Le Gentil found that war had broken out between England and France, and the British had captured Pondicherry. Worse, storms blew his ship off course, resulting in weeks spent wandering the Indian Ocean and the Arabian Sea. On the day of the transit, he was nowhere in sight of land. He was unable to make any of his measurements accurately from the deck of a rolling ship.
Rather than returning to France in defeat, Le Gentil resolved to try again for the next transit eight years later. He set sail for Manila in the Philippine Islands, where his calculations predicted the conditions would be best for the transit of 1769. Again he was beset by misfortune. The suspicious Spanish governor there accused him of forging his letters of introduction and made it clear he was not welcome. Sailing once more for Pondicherry, now back under French control, he built his observatory and spent the time until the transit studying the local flora and fauna and Indian astronomy. Unfortunately, a storm arose on the long-awaited day of the transit, and once more Le Gentil missed everything. (The weather was perfectly clear back in Manila.) Dejected, Le Gentil packed up his samples and returned to France, but on his way home he came down with dysentery, and his ship was wrecked in a hurricane. By the time he finally reached France he had been away for eleven years, six months, and thirteen days. His heirs had declared him dead and fought over his estate. His wealth was gone, the specimens he sent from India never arrived, and his chair in the French Academy of Sciences had been bestowed on another man. According to the Canadian astronomer Helen Sawyer Hogg, who translated Le Gentil’s journals, his voyage “is probably the longest astronomical expedition in history. In fact, it is quite possible that, except for interplanetary travel, there will never be astronomical expeditions to equal in duration and severity those made for that particular pair of transits.”
One hundred and five years later, when the next two transits occurred, the world was better prepared than before. In 1874, Russia would launch twenty-six transit expeditions, Britain twelve, the United States eight, France and Germany six each, Italy three, and Holland one. Every country with scientific aspirations joined in the worldwide endeavor. In the United States, the composer John Philip Sousa even composed the Transit of Venus March.
From all these transit expeditions, astronomers calculated a distance for the AU of 92,885,000 miles, within 0.07 percent of what we now know to be the true value. This one number, acquired from observations made at far-flung points around the globe, at last revealed the extent of the solar system and each planet’s size—and the numbers were huge. But let’s bring these literally astronomical dimensions to a more manageable scale. Imagine we were to shrink the solar system to the size where the Sun was a grapefruit, 5 inches (10 cm) in diameter. On this scale, the Earth, no bigger than a tiny candy sprinkle 1 millimeter in size, is 16 yards (or meters) away. Jupiter, a pebble a half inch in size (1 cm), is another 60 yards away. Pluto and the Kuiper Belt, at the edge of the observable solar system, are grains of sand a third of a mile (500 meters) away from our tiny planet.
In 2006, NASA launched the New Horizons spacecraft, the fastest machine ever created; even so, it took nine years to reach Pluto. Nine years to travel the third of a mile in our model solar system. Yet even light, the fastest thing in the universe, still takes almost five hours to travel that distance. The speed of light may be fast, but it isn’t infinite. For centuries, philosophers debated whether light even had a speed. Some, like Alhazen in his treatise on optics, said it did, arguing that nothing could be in two places at once. Others argued that it did not. The French philosopher René Descartes wrote that if light took time to travel during a lunar eclipse, then it would take time for the Earth’s shadow to fall upon the Moon, and it would take an equal amount of time for the sight of it to travel back to us. By the time we saw the eclipse occur it might already be over, and surely such a thing was impossible. Descartes was wrong, of course. Or rather, he was right; he just had the wrong moon. To truly see this effect, he needed to look for an eclipse around another planet.
In 1676, Ole Rømer’s job was to measure the period of Jupiter’s moon Io for the task of mapping France. Rømer, who was Danish, found that rather than having a constant rate, the time between Io’s eclipses grew less as the Earth approached Jupiter and longer as it traveled away. This is so because while the Earth approaches Jupiter, the light from each little eclipse travels progressively shorter distances to reach us. The time between eclipses appears to shrink. As the Earth moves away from Jupiter, the distances increase, and the “news” of each eclipse has farther to travel. The period lengthens. The maximum difference in distance is the diameter of the Earth’s orbit (twice an AU). From the maximum difference of 22 minutes in Io’s orbital period, Rðmer found that light requires roughly 11 minutes per AU. This is terribly fast from our terrestrial experience, but not instantaneous.
Our measurements have improved for both the speed of light and the distance to the Sun. We now know the Earth lies 8.3 light-minutes from the Sun. We see the Sun as it was 8.3 minutes ago, and we see Pluto as it looked almost 5 hours ago. But how far is it to the stars? Once the length of the AU was known, astronomers could use the parallax of nearby stars to reveal their distance. The closer a star, the more its position moves back and forth against the distant stars each year (remember the motion of your thumb when looking with one eye then the other). Because even the closest stars are so distant, the first parallax motion was not observed until 1838, long after anyone still needed proof that Copernicus had been correct that the Earth really moved around the Sun. The star was 61 Cygni, and its parallax shift was only 1/12,000th of a degree, near the limit of what even telescopes today can measure without being sent into space. The result places 61 Cygni almost 658,000 AU away from the Earth, meaning the light we see from it tonight took 10.3 years to travel here. On the scale of our model solar system, 10.3 “light-years” is the distance between Los Angeles and Paris across the surface of the globe. For comparison, the farthest human beings have ever traveled into space is the orbit of the Moon, no bigger in our model than the width of a thumbnail.
For more than a century now, astronomers have used (and built upon) this technique to erect a distance ladder out into the universe, where each step is based upon the one before. From the distance between cities in Egypt, we measure the circumference of the Earth and the locations of explorers around the globe. Their positions reveal the distance to Venus and the Sun, from which we calculate the vast distances between stars. From these distances we learn that we live in a vast spiral disk more than 100,000 light-years across containing more than 200 billion stars. In all that vastness, are there other worlds around other suns? Less than a quarter of a century ago, anyone asking how many planets there were would have been told nine (including Pluto). The number of solar systems was one: ours. Today, we know there are thousands of planets around other stars with hundreds of multi-planet solar systems. New ones are being discovered so rapidly that before I could write an exact number, it almost certainly would be out of date.
The technique that has discovered the most planets depends upon detecting their silhouettes as they pass in front of their Suns. In essence, we look for their shadows during momentary eclipses (or, more precisely, transits). The first transiting exoplanet (the term used for planets orbiting other stars) was discovered in 1999 around the star HD209458. For three hours every 3.5 days, it blocks enough light from its star that astronomers on Earth can tell it is 2.5 times larger than Jupiter in size.
In 2009, NASA launched the Kepler Mission, a space telescope designed specifically to look for transits around stars in a tiny patch of the sky toward the constellation of Cygnus. Kepler has found thousands of planets with thousands more waiting to be confirmed. While many of these are the size of Jupiter and Saturn, astronomers poring through the vast backlog of data are finding increasingly smaller planets. In 2015, one was found that was dubbed “Earth 2.0,” a planet only 60 percent larger than the Earth in an orbit about the same as ours around a sun only slightly brighter than our own. The successor to Kepler is the Transiting Exoplanet Survey Satellite (TESS), set to launch in 2017, the same year as the Great American Solar Eclipse. Unlike Kepler, it will search the entire sky for transiting planets. When TESS finds candidates, NASA will look for ground-based observatories to conduct follow-up observations. The technology needed to do so is now available to small universities and even amateur astronomers. My students and I, using a telescope no bigger than the first one my father bought me when I was a boy, have watched a planet three times the mass of Jupiter orbiting its star every three nights, placing us briefly in a shadow more than three hundred light-years long.
The question now is whether any of these planets are suitable for life. One benefit to transits is that at the moment a planet passes in front of its star, some tiny portion of the starlight we see has filtered through the planet’s atmosphere. Atoms in the planet’s atmosphere absorb a tiny amount of this starlight, with each element absorbing a different combination of colors. The starlight we receive during a transit therefore bears the chemical fingerprints of the gases in the planet’s sky. We are no longer simply discovering other worlds; we are learning the compositions of their atmospheres. We are, in effect, sniffing alien air.
Astronomers applied this technique to our own world. Observing our Moon during a total lunar eclipse, they were able to detect the chemical signature of oxygen, ozone, and water vapor in sunlight that had filtered through our own atmosphere before falling upon the lunar surface. Together, these molecules mark our planet as an abode for life. It’s only a matter of time before we detect the same combination from planets around other stars.
That is where our steps through the universe have led: from the shores of the Peloponnese to the sands of the Nile, from the islands of the Caribbean to the village of the Mandan, and from Tahiti to the Moon, the Sun, and the inky depths of the sea of space beyond.
Someday, maybe farther in the future than Anaxagoras is in our past, the first ships will set sail for the stars. When they do, the stars that are their destinations will be the ones with worlds discovered during this generation. And they will have been discovered the way we have discovered the universe around us, by following the light and shadows of distant worlds.