Finding Our Place in the Galaxy Took a Good Bit of Math and Ingenuity
If you were to take a test right now, would you be able to find yourself on a map of your city? Your country? A map of the world? Many of us probably could, but what if you didn't have a map to work with?
Figuring out where you are in a space whose dimensions you do not know is a tricky thing. Without an external point of reference, the best you can determine is where you are relative to other objects, like that tree or that street or building.
In the case of our planet and solar system, how do we know where we are in the Milky Way, the galaxy we call home?
This isn't a problem unique to astronomy, the challenge of mapping and localizing oneself in an unknown space has been a difficult challenge for explorers throughout history navigating the unknown.
"Finding one's location in a cloud of a hundred billion stars—when one can't travel beyond one's own planet—is like trying to map out the shape of a forest while tied to one of the trees," said Laurence A. Marschall of Gettysburg College in Gettysburg, Pennsylvania.
And given our propensity to put ourselves at the center of everything, figuring out where we really are can be more challenging still. But techniques passed down through time have given us a useful set of tools that have helped us map the stars and find our place in the cosmos.
Early attempts at placing ourselves in the universe
Ancient astronomers used the changes in the night sky and cycle of the sun to come up with the first attempts at placing our position in the universe, and they decided that we were clearly at the center of everything.
While this seems silly to us now, at the time, it was an easy mistake to make. The stars and constellations, with their regular, unchanging patterns, swept across the night sky and shifted with the seasons, always returning to where they started from with regularity in a cycle, known as sidereal time.
Both the sun, the most prominent of celestial bodies, and the moon, the second-most, appear to orbit the Earth, helping give rise to the geocentric model of the universe that most people believed in until the middle of the second millennium CE, and even then for a long time after in some quarters.
But even in ancient times, there were clear indications that the Earth wasn't at the center of things.
For one, the visible planets that "wandered" across the backdrop of seemingly fixed stars or near the sun during the twilight of dawn or dusk do not obviously orbit the Earth. In fact, Mercury and Venus both cycle through a noticeably fixed space in the sky and can be observed orbiting the sun.
Aristarchus of Samos, an ancient Greek philosopher who lived from about 310 to 230 BCE is credited with proposing the first heliocentric model of the universe, and he did a very good job of it.
Aristarchus correctly stated that the Earth orbits the sun, the soon orbits the Earth, as well as determining the correct order of the five visible planets relative to Earth, with Mercury and Venus orbiting nearer to the sun with Mars, Jupiter, and Saturn orbiting beyond Earth's orbit.
For philosophical reasons, rather than scientific ones, this model was rejected for two millennia, until Galileo demonstrated that Jupiter is orbited by four moons, directly contradicting the idea that the Earth was the center of everything.
During the Scientific Revolution, astronomers were able to determine that the stars in the night sky weren't fixed points on a celestial sphere that marked the boundary of the universe, but other suns like our own located at incredible distances from us.
We didn't know that there were even galaxies. Given the telescopes of the era, galaxies were indistinguishable from other clouds and nebula in the night sky, and it wasn't until the 1920s that instrumentation allowed galaxies to start to take shape. Edwin Hubble, working with the most advanced telescope of the era, calculated that the Andromeda Nebula was 900,000 light-years from us (he was off by half, as the actual distance is closer to twice what Hubble calculated), and at a distance so great that it had to represent an entirely separate galaxy of clustered stars.
With definitive evidence of another galaxy, the obvious question presented itself: If we are in a distinct galaxy, what does it look like, and where is our solar system located within it?
Mapping the Milky Way
Knowing the rough shape of our galaxy is an important step in building a map that we could use, but how can you tell the shape of the galaxy from the inside?
Fortunately, we had some solid evidence to work from. With documented observations of Andromeda's spiral structure going as far back as 1850, the dazzling nebula of dust and stars in the night sky that gives our galaxy its name, the Milky Way, provided one of the best pieces of evidence that our galaxy is not that different from Andromeda.
"One gets a rough idea of the shape of the Milky Way galaxy by just looking around—a ragged, hazy band of light circles the sky," Marschall said. "It is about 15 degrees wide, and stars are concentrated fairly evenly along the strip. That observation indicates that our Milky Way Galaxy is a flattened disk of stars, with us located somewhere near the plane of the disk.
"Were it not a flattened disk, it would look different. For instance, if it were a sphere of stars, we would see its glow all over the sky, not just in a narrow band. And if we were above or below the disk plane by a substantial amount, we would not see it split the sky in half—the glow of the Milky Way would be brighter on one side of the sky than on the other."
Knowing that ours is a spiral galaxy like Andromeda and countless others we've discovered in the night sky over the years, we had a good template to work with for building out a rough map of the Milky Way. But how did we figure out where we were on that map?
Figuring out where we are in a spiral galaxy is definitely more difficult than knowing that we were in a spiral galaxy, but it isn't impossible.
"The position of the sun in the Milky Way can be further pinned down by measuring the distance to all the stars we can see," Marschall said. "In the late 18th century, astronomer William Herschel tried to do this, concluding that the earth was in the center of a 'grindstone'-shaped cloud of stars. But Herschel was not aware of the presence of small particles of interstellar dust, which obscure the light from the most distant stars in the Milky Way.
"We appeared to be in the center of the cloud because we could see no further in all directions. To a person tied to a tree in a foggy forest, it looks like the forest stretches equally away in all directions, wherever one is."
It wasn't until we developed better instruments though that we could get a better sense of the galaxy's borders, as well as where its center might be located. We've known for a long time now how to measure the distance to nearby stars using stellar parallax (the difference in direction of a celestial object as measured from two widely separated points).
This helped Harlow Shapely, an American astronomer in the early 20th century, determine the distance to several globular clusters of stars which covered a span of about 100,000 light-years in diameter, and they appeared to be centered around a point in the constellation Sagittarius.
"Shapley concluded (and other astronomers have since verified) that the center of the distribution of globular clusters is the center of the Milky Way as well," Marschall said, "so our galaxy looks like a flat disk of stars embedded in a spherical cloud, or 'halo,' of globular clusters.
"In the past 75 years, astronomers have refined this picture, using a variety of techniques of radio, optical, infrared, and even X-ray astronomy, to fill in the details: the location of spiral arms, clouds of gas and dust, concentrations of molecules and so on."
This all lets us know that we are at least a decent distance away from the center in one of the galaxy's arms. And since we can clearly see another galactic arm opposite Sagittarius, we're located on an inner arm of the Milky Way.
What's more, as we've observed this apparent center more, we've been able to home in on the galactic core: the supermassive black hole in the center of the galaxy known as Sagittarius A*. Knowing the location of Sagittarius A*, we can measure our distance from it, and at least come up with our relative position.
"The essential modern picture is that our solar system is located on the inner edge of a spiral arm, about 25,000 light-years from the center of the galaxy, which is in the direction of the constellation of Sagittarius," Marschall said.
Where we would be located in the Milky Way to an observer outside the galaxy is another matter, as it depends entirely on that observer's location outside our galaxy. But it's still the best estimate we're going to have for a while, and it's more than enough for our purposes.
In many ways, figuring out where we are in the universe might be an easier task, especially since we're looking for our position relative to objects outside our own galaxy.
And once the James Webb Space Telescope comes online, we can start identifying the edges of the observable universe by identifying the distribution of the oldest galaxies and stars in the night sky.
That is at least a few months off, however, but soon we may take the next big leap into truly identifying our place in the larger cosmos.