Discovering the Position of Earth Within the Milky Way

For many centuries, humans held a simplistic view of the universe, perceiving Earth as the central point of all existence. They believed that celestial bodies like the sun, stars, and other cosmic entities revolved around our planet in perfect harmony. This perspective arose from our inherent tendency to place ourselves at the center of everything.

However, as human understanding of the cosmos evolved, a journey unfolded that eventually revealed our true location within the vast Milky Way galaxy. This voyage of cosmic discovery is a captivating narrative filled with determination, innovative thinking, and ground breaking scientific achievements.

To accurately pinpoint our position within the Milky Way, it is essential to grasp the galaxy's actual structure. The night sky provides the initial clues, presenting us with a faint, luminous band of stars, a celestial ribbon recognized as our galaxy. This band-like formation suggests that we inhabit a flattened disc rather than a spherical structure. If our galaxy were spherical, we would observe a more evenly dispersed distribution of stars across the entire sky. Moreover, the band appears to divide the night sky, indicating that we exist within its plane, neither above nor below it.

However, leaving this plane to gain a complete perspective of the Milky Way's shape remains impractical. Such a journey would entail traveling hundreds or possibly thousands of light years perpendicular to this plane. To provide context, consider Voyager 1, the most distant human-made object, which, despite nearly half a century of space travel, has covered only a distance equivalent to 0.02 light years. This underscores the immense distance required to fully observe the Milky Way's structure.

Nonetheless, the lack of technology for direct observations did not deter us from determining our galaxy's shape and our position within it.

The first significant stride in this journey occurred in the 18th century, courtesy of William Herschel, renowned for discovering Uranus in 1781. Herschel was not merely an astronomer but also a cosmic cartographer. Armed with his handcrafted telescope, which surpassed any predecessors in peering into the cosmos, he embarked on an audacious mission: mapping the stars of the Milky Way.

In the late 1700s, with the assistance of his sister Caroline, Herschel divided the night sky into around 600 zones. With unwavering patience and precision, he catalogued the position and brightness of every observable star within these areas. His method involved observing the night sky through his telescope and marking the stars' relative positions on a grid. His meticulous star count led him to propose that our galaxy had a disc-shaped structure.

Herschel's observations also hinted that our solar system was near the center of this structure. However, his map was not entirely accurate because he was unaware of interstellar dust that obstructed our view of stars in the Milky Way's central region. Consequently, Herschel's map depicted the central region as less dense than it truly is. Despite its inaccuracies, his map marked a significant leap in comprehending the Milky Way's structure, being the first to portray it as a disc and providing crucial insights into our galaxy's configuration.

Fast forward from the 18th century to the 20th century, and we find ourselves at the Harvard College Observatory, where a curious astronomer with a profound fascination for the universe is witnessing something extraordinary. Her discovery is poised to revolutionize astronomy and alter its trajectory permanently.

In the realm of cosmic discovery, it is often said that every star, celestial body, and pioneering mind plays a role. Henrietta Swan Levitt, an American astronomer, is one such luminary who made substantial contributions to our understanding of the cosmos, consequently informing our knowledge of our place within the Milky Way.

Levitt's focus centered on a unique type of star known as Cepheid Variables. These stars possess a peculiar property: they pulsate, alternately growing brighter and dimmer at regular intervals. Levitt's role at the Harvard College Observatory involved cataloguing these stars, a task that led her to a ground breaking discovery.

She meticulously catalogued 1,777 variable stars. In 1908, she made a pivotal observation: a direct relationship between the luminosity of these stars and their pulsation periods. Brighter Cepheids pulsated more slowly than their fainter counterparts. This seemingly simple yet profound relationship, now recognized as Levitt's law, enabled astronomers to determine the distances to Cepheids by measuring their pulsation periods.

Cepheids became established as the standard candles for measuring astronomical distances, providing astronomers with a promising method capable of measuring distances up to 20 million light years. But how does this help us understand our position in the Milky Way?

To answer this question, we must introduce another key figure, Edwin Hubble. Before the 1920s, many scientists believed that the Milky Way was the sole galaxy in the universe. While images of neighboring galaxies like Andromeda had been captured, they were assumed to be star systems within the Milky Way. Consequently, most astronomers regarded the Milky Way as the entirety of the universe.

However, as telescopes improved, some began to question this theory. They noticed an increasing number of hazy patches in the sky that did not resemble typical star-forming regions within our galaxy. Measuring the distances to these stars was a significant challenge, as stars vary widely in size and brightness.

Henrietta Swan Levitt's work became indispensable. She had demonstrated that Cepheid variables could be used to determine distances to galaxies up to 20 million light years away. Inspired by her work, Edwin Hubble decided to measure the distance to the Andromeda galaxy by studying a Cepheid variable star within it.

His findings stunned the scientific community: Andromeda was approximately 930,000 light years away, far beyond the confines of the Milky Way. Although this distance estimate differs from the current value, Hubble's discovery confirmed Andromeda's location outside the Milky Way, reshaping our understanding of the universe.

The revelation that Andromeda resembled a flattened disc of stars akin to the Milky Way sparked a question among astronomers: where are we situated within our own galaxy? This is where the work of American astronomer Harlow Shapley becomes pertinent.

Initially, Shapley believed that everything visible to us was part of the Milky Way—an assumption that Edwin Hubble's research later disproved. Recognizing his error in the early 1920s, Shapley adjusted his perspective and embarked on an ambitious effort to catalogue galaxies, documenting as many as 1,249 in just six years.

However, Shapley's work from 1914 to 1918 played a pivotal role in our quest to determine our position in the Milky Way. Armed with the powerful Mount Wilson Observatory's 60-inch telescope, the most advanced instrument of his time, Shapley focused his research on globular clusters—dense groups of hundreds of thousands or even millions of stars that gravitationally coalesce to form spherical shapes, typically found above or below the galaxy's plane.

These globular clusters are some of the universe's oldest objects, dating back over 10 billion years. Shapley's observations revealed that these clusters were arranged in a spherical pattern around the galactic core, particularly in the direction of the Sagittarius constellation.

This observation held great significance. If we were near the Milky Way's core, we would observe globular clusters scattered across the entire sky. However, most of them are visible primarily near the Galactic Center, in the direction of the Sagittarius and Scorpius constellations. This suggests that we are not near the center of the Galaxy, as William Herschel had hypothesized, but rather positioned toward the outer arm.

Nonetheless, Shapley's calculations slightly overestimated our actual distance from the galactic center, placing us somewhere between 33,000 and 90,000 light years away. With advancements in modern astronomy, we have since refined this figure, locating our solar system near a small partial arm known as the Orion arm or Orion spur, situated between the Sagittarius and Perseus arms, approximately 26,000 light years from the Milky Way's core.

The Milky Way serves as our home—an elegant spiral of starlight whirling in the cosmic abyss. However, understanding its grand design, vast composition, and our position within it has often been obscured by the monumental task at hand, until now.

Enter Gaia, the European Space Agency's celestial cartographer—a silent voyager sailing through the cosmic sea, revolutionizing our comprehension of the Milky Way. Launched in late 2013, Gaia embarked on an ambitious mission: to craft the most comprehensive and detailed 3D map of our galaxy ever conceived.

Armed with the task of surveying approximately 1 billion stars, which represents about one percent of the stars in our galaxy, Gaia has been diligently charting the cosmos. This mission enables astronomers to study the Milky Way as a whole, uncovering its structure, dynamics, and evolution.

Gaia provides insights into the galaxy's overall structure, including star distribution, spiral arm arrangement, and the characteristics of the galactic bulge and halo. This perspective refines our understanding of our position within the Milky Way and how our galaxy fits into the broader context of the local universe. Ultimately, the Gaia mission represents a significant leap in astrometry, the study of celestial bodies' positions and motions.

While not as famous as the Hubble Space Telescope or the James Webb Space Telescope, Gaia is an astronomical game-changer, silently revolutionizing our grasp of the Milky Way. The precise and comprehensive data collected by Gaia serves as a valuable resource that will continue to drive astronomical discoveries for decades to come, propelling our cosmic journey deeper into the universe's mysteries.

In recent developments, astronomers have noted that a black hole has abruptly changed its direction, now pointing towards us—a captivating addition to our ongoing exploration of the cosmos.