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Why Do Cosmic Distances Have So Many Units?

Imagine how confusing it is to deal with distances measured in redshift if you thought light years and parsecs were confusing.

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The units that make sense to people who are not astronomers differ from how scientists typically describe astronomical distances. As Douglas Adams so eloquently explained, space is a very large place. Thus, large units are required to adequately describe it. If you really want to measure the distance in centimeters between Earth and the Andromeda Galaxy, the result is a lot of confusing and meaningless zeros at the end.

The most widely used unit of measurement for cosmic space among the general public is light years. This is the distance light will travel through a vacuum in the time it takes for the Earth to orbit the Sun, as the name would imply. True, most people cannot even begin to imagine it, but at least the name is well-known. Furthermore, it provides a sense of scale when discussing more distant stars because the star that is closest to us, aside from the Sun, is just over four light years away.

The galaxy is described as being “A hundred thousand light years side to side” in Monty Python’s The Meaning of Life, which also serves as a rather helpful introduction to the concept of light years. Even after some uncommon outlying stars were found, the number was not only fairly accurate at the time, but it hasn’t been significantly altered by further research.

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Light years also provide us with instantaneous information about how long light has taken to reach us, which is fascinating if not always helpful. We therefore don’t need to perform any additional calculations to determine that a supernova we recently observed exploded 21 million years ago when we state that it is 21 million light years away.

In fact, light years are so helpful that their derivatives can be used to measure distances such as light hours or light months, which represent the separations between stars and the Voyager spacecraft, respectively.

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At IFLScience, we most frequently utilize this unit for astronomical stories. But light years are hardly mentioned in the original scientific papers that we are reporting on.

Rather, redshift (z), parsecs (pc), and astronomical units (AU) are the three most commonly used units for astronomical distances. It’s helpful to understand what each of these means because both we and other popular science websites will occasionally use them, especially AU.

But before we get started, it’s important to note how Earth-based each of these is. Apart from redshift, no extraterrestrial society would utilize these units since their planet would orbit their star at a different distance and take a different amount of time. Put differently, our understanding of the universe is shaped by our local circumstances. Our perspective of the universe is often influenced by our background and is rarely objective.

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Distances within the Solar system and occasionally between other star systems are measured using astronomical units. The average distance, measured in astronomical units (AUs), between Earth and the Sun is 150 million kilometers, 93 million miles, or 8.3 light minutes. This has several benefits. Even though the majority of us aren’t exactly sure how far we are from the Sun, it’s still helpful to know to determine whether an object in passing poses a threat or to comprehend why some planets are significantly hotter or colder than others. In the meantime, light years are too big to adequately convey these kinds of distances; it would be like attempting to measure the width of your finger with miles.

The unit of parsec is far less intuitive. A star needs to be one parsec away to exhibit a parallax of one arcsecond. In other words, when the Earth moves between the two ends of its orbit, it appears to move by a mere 3,600th of a degree relative to farther-off stars.

That’s hardly an eloquent explanation, and it’s not immediately clear why this unit is superior to the light year. It’s not like a parsec is sufficiently larger or smaller to be a much better way to describe certain distances—one parsec is equal to 3.26 light years. The original Star Wars movie seemed to use parsecs as a measure of time rather than distance because they are so poorly understood. Fortunately, a very clever explanation has since been developed to save face.

Nevertheless, the majority of papers use units such as parsecs for nearby stars, kiloparsecs for galaxy distances, and megaparsecs for distances between nearby galaxies. All you have to do to convert to light years is divide by 3.26.

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Astronomers appear to favor parsecs more due to habit than any practical benefit over light years. Cynically speaking, some might contend that it’s a holdover from the days when it was thought beneficial to use jargon that excluded non-specialists rather than a barrier to the general public’s appreciation of science.

Redshift is still the most perplexing unit of all, but the reasons for using it are becoming more clear.

Redshift is an outcome of the universe’s expansion. Galaxies separate from one another as the universe expands, shifting the light from them to longer wavelengths. This shift is slight for nearby galaxies and can be dominated by localized factors, leading to the possibility that some galaxies are even traveling in our direction (or, more precisely, our direction toward them).

Nevertheless, at larger separations, the wavelength will be redshifted, or lengthened, in proportion to the galaxy’s distance. The Hubble Constant, an as-yet-uncertain measurement, is what determines how exactly redshift and distance relate to one another. It makes sense to describe a galaxy in this way rather than its distance in light years or parsecs since in most cases it is the redshift we can measure. There would be errors in any translation.

Even more perplexingly, it is not simple to convert redshift into light years. It is predicated on a multitude of universe-related assumptions, not all of which are accepted. There are various online calculators available, and because they view the results differently, they don’t always yield the same results.

Because of this, there are multiple levels of uncertainty in the distances mentioned in popular science articles regarding galaxies from the early universe.

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I'm Michael, a young enthusiast with an insatiable curiosity for the mysteries of science and technology. As a passionate explorer of knowledge, I envisioned a platform that could not only keep us all informed about the latest breakthroughs but also inspire us to marvel at the wonders that surround us.
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