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One of the greatest difficulties when discussing the physical world is conveying its immense scale. While we can estimate the number of molecules contained in a single drop of water (roughly 1.5 sextillion, 1,500,000,000,000,000,000,000) or measure the distance light traverses in a single second (around 300 million meters), the values we obtain are so alien that we cannot intuitively comprehend them. For most people, the difference between one and 10 is far more palpable than the difference between a thousand billion and a thousand trillion. Against our Earth-bound frames of reference, sizes and distances grow too large or small, speeds and intervals too fast or slow, forces too strong or weak. Few disciplines illustrate this more clearly than astronomy, the oldest of the natural sciences.
Consider this: We live in a universe so large that light itself (and nothing goes faster) takes years to travel between stars, eons to travel between galaxies. All we see in the sky are essentially old photos of celestial objects as they were when their light first left to travel to Earth. When we look across space, we also look back in time.
The Sun is just a yellow dwarf star, and, like most stars visible in Earth’s night sky, is contained in the Orion spur, a diminutive tendril of star-forming gas and dust sandwiched between the outskirts of two of the four massive spiral arms that make up our Milky Way galaxy. The Milky Way is roughly 100,000 light-years wide, a few thousand light-years thick, and filled with hundreds of billions of stars, each of which may have its own accompanying retinue of planets. Our closest major galactic neighbor, another spiral galaxy named Andromeda, lies 2.5 million light-years away—but it’s getting closer by some 120 kilometers each second, all the time.
Some 3 billion years in the future, Andromeda may crash into the Milky Way, forming a single giant galaxy that will dominate all the other objects in our cluster of galaxies, which is known as the Local Group. We can observe billions and billions of galaxies further away, but even all this may be but an infinitesimal part of the larger universe, which seems to have sprung into existence 13.7 billion years ago. The universe is expanding, and even accelerating in its expansion, and may continue to do so forever.
Though this is just a cataloging of objects, distances, and sizes, the numbers involved lend the list a feeling of grandeur and raise questions in the curious mind: Just how is it that we know the distance from the Earth to the Sun, the other planets, and faraway stars? How do we know the architecture and future of our galaxy or the expansion rate of the universe? The short answer is that we know these things because of the cosmic distance ladder, a suite of interdependent methods to measure successively greater distances in the universe. Though most of the ladder was created in the 20th century, millennia of effort have contributed to its construction, and it is still being refined.
A dearth of records limit our knowledge of astronomy’s earliest era, and ancient astronomers were themselves hindered by the absence of telescopes, but it is clear they took the first steps in establishing cosmic distances. Astronomy in antiquity blossomed with the Babylonians, but reached its zenith in Hellenistic Greece, where Eratosthenes of Cyrene calculated the circumference of the Earth, and Aristarchus of Samos proposed that the Earth revolved around the Sun.
Based on Aristarchus’ ideas, the Greek mathematician Archimedes wrote The Sand Reckoner, a work where he attempted to estimate the universe’s size and how much sand would be required to fill it. Archimedes assumed the universe was a sphere, and his estimation of its diameter corresponds to a modern measurement of about one light-year. He thought it could hold about 1063 sand grains. Imperfect as it was, the astronomy of the Greeks would not be surpassed for more than a thousand years.
In 1543, Aristarchus’ theory of heliocentrism was revived and expanded by the Polish astronomer Nicolaus Copernicus in Renaissance Europe. Then, building on the work of Copernicus, as well as the telescopic observations of Galileo Galilei and Tycho Brahe, the German astronomer Johannes Kepler devised his three eponymous laws of planetary motion. Kepler’s third law established a clear relationship between the period of a planet’s orbit and its distance from the Sun. By observing the motions of the planets from night to night, Kepler could estimate, for instance, that Mars was 1.5 times more distant from the Sun than Earth was, and that Jupiter was five times more distant still. But without the calibration of knowing precisely how far the Earth was from the Sun, such estimations were of limited use.
Today astronomers use radar beams to measure interplanetary distances, aiming powerful pulses at a planet or moon and waiting for the reflected “echo” to return. But before radar, such measurements were much more difficult. Astronomers relied on something called parallax. Extend your arm and look at your thumb first through your left eye, then your right. You’ll notice your thumb’s apparent position will change as you switch back and forth between eyes. This displacement is caused by the difference in perspective provided by two spatially separated viewpoints; the closer an object is to the two observation points, the greater that object’s parallax. Using the principle of triangulation, an observer can calculate the distance to an object using the object’s observed parallax and the known distance between the two observation points.
The Italian astronomer Giovanni Cassini, along with his colleague Jean Richer, performed the first measurement of interplanetary parallax in 1673. The planets were so distant that they only yielded clear parallax shifts when observers were located on far-flung portions of the globe. Cassini observed the position of Mars in the sky above Paris, France; Richer observed Mars’ position above Cayenne in French Guiana. By calculating the difference between the two measurements, Cassini estimated the Earth–Mars distance to within 10 percent of its known modern value. Parallax measurements of other interplanetary distances soon followed. By knowing these, the Earth’s distance from the Sun could finally be estimated with reasonable certainty.
Parallax worked well for interplanetary distance measurements because of the background of fixed stars, which allowed small shifts in planetary positions to be seen. But what was a boon for determining interplanetary distances was a bane for finding interstellar ones. No one knew anything of great certainty about interstellar distances, other than that they were large enough to make stars seem to hang immobile in the sky.
Soon, clever astronomers began attempting to measure stellar parallax by exploiting the Earth’s motion around the Sun. If you measure the position of a nearby star in the sky in January, then when you measure that star’s position again in June, the Earth’s orbit around the Sun will have created a difference of hundreds of millions of kilometers between your two observations, enough to reveal a nearby star’s parallax.
The question became, which stars are nearby? Fortunately for 19th century astronomers, improved record keeping and telescopic observations revealed that stars aren’t actually “fixed.” In fact, several stars were eventually found that almost imperceptibly crept across the sky over timescales of months and years. These slight motions, it was hoped, indicated that those stars were relatively close to us.
Between 1832 and 1833, the Scottish astronomer Thomas Henderson measured the first reasonably accurate stellar parallax based on his observations of the Alpha Centauri star system. His calculations indicated Alpha Centauri was about 3.25 light-years away. (The modern estimate, using parallax, is 4.39 light-years.) But Henderson’s uncertainty about the validity of his measurements caused him to delay publicizing his findings. Henderson finally published in 1838, two months after another astronomer, Friedrich Bessel, announced his own parallax measurement of a slightly more distant star, 61 Cygni. Today Bessel is remembered as the first to measure stellar parallax, and the technique is still the baseline for obtaining cosmic distances.
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