From the March 2013 issue

Hubble and the light-year

May 2013: Why we should stick to more understandable distances.
By | Published: March 25, 2013 | Last updated on May 18, 2023
How can we grasp the enormity of space? One thing is sure: We should at least make our units of measurement as intuitive and friendly as possible.

We often fail. Take, for example, cosmic expansion, the Hubble constant. In October, the highly respected Carnegie Observatories determined that its value is 74.3 ± 2.1 kilometers per second per megaparsec. The science media reported this verbatim.

But that doesn’t help us understand what’s going on. The problem is twofold. Many of us think in miles, not kilometers, and the megaparsec is even less meaningful. Have you ever heard anyone say it, even on TV? It would be easy to translate that key constant into audience-friendly language, but, strangely enough, no one does so.
Let’s start by seeing why the parsec has become the professional astronomer’s favorite distance unit. Bear with me: This isn’t pleasant for math-o-phobes.
Earth’s yearly path around the Sun makes nearby stars appear to shift back and forth against the starry background. We can determine distance by carefully measuring such vacillations; this is trigonometric parallax. Stars shift less than a second of arc, a tiny angle equal to the width of a U.S. quarter seen from 3 miles (5km) away. Turns out, we’d see a parallax shift of exactly 1 arcsecond if a star were 3.26 light-years away. That distance was named the parsec, an abbreviation for “PARallax of one SECond of arc.” It has a cool high-tech ring we science geeks like.

Astronomers can derive a star’s distance in parsecs by measuring its annual parallax and dividing it into 1. For example, Vega’s parallax is 0.13 arcsecond, which divided into 1 yields a distance of 7.69 parsecs. So the parsec is a logical unit in the field of astrometrics, which is able to directly measure the 118,000 nearest stars.

Past a few thousand light-years, however, parallaxes get too small. So this distance-finding method works for fewer than one in a million of our galaxy’s stars and for none of the 125 billion known galaxies. For virtually the entire universe, you have to find distance by another method and then express it using whatever unit you wish — parsecs, light-years, furlongs, it doesn’t matter. Parsec offers no advantage.


Astronomers attach prefixes like kilo- and mega- to the word to talk about objects thousands or millions of parsecs away. Thus, we’ve now arrived at that megaparsec term, invariably used when citing the Hubble constant.

Here’s why light-year and million light-years are superior. Unlike with the parsec, there are no angular measurements involved and no math derivations. Light is familiar and fast. A photon could make almost 72 trips from Boston to Hollywood in a single second. Even at that speed, light takes roughly an hour to get here from Saturn. Thus, Saturn is about 1 light-hour from us. Simple. Light’s travel-distance in a year is, of course, a light-year. It’s huge and yet intuitive. That its speed never varies in the emptiness of space is icing on the cake and provides a valuable, rare commodity: a constant.

The nearest spiral galaxy, Andromeda, lies 2.5 million light-years away. We can make sense of this several different ways. We might note that when we observe Andromeda, its light began its journey 2.5 million years ago, just as early hominids were learning to stand upright when eating their sushi. Or we could say that if our technology ever creates rockets that can zoom at nearly light speed (today’s best are 18,000 times slower), we could reach Andromeda after 2.5 million years of travel. Either way, the true enormity of its distance hits home.
All such musings spring to mind simply by hearing the galaxy’s remoteness expressed in light-years. No analogies or appreciations pop up if we instead cite its distance as 767 kiloparsecs.

Light has gifted the human mind with an easy unit for expressing large distances. Let’s now use it for the Hubble constant. According to the newest data, the universe’s rate of expansion is 14 miles per second faster for each million light-years of distance. Thus, a galaxy 100 million light-years away zooms 1,400 miles a second.
Think of a city 1,400 miles from you. Visualize going there in one second. That’s the speed a galaxy races away if it happens to lie 100 million light-years from us. Voilà, you’ve grasped the cosmic expansion rate. (If you prefer metric, the Hubble constant is 23 kilometers per second per million light-years.)

Now try using 74 kilometers per megaparsec. Nothing clicks.

I probably sound like a lunatic on a soapbox, cursing the parsec and cooing with unholy adoration for the light-year. I don’t care. We should always use light-years for stuff like this. It’s just too sensible to abandon.

Join the crusade. Save the light-year.

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