Why do scientists use specialized units for distance when metric units are perfectly adequate?

Question

Three super common measures in space distance are the AU, the parsec and the light year. Although I understand the origins of these units, it seems odd to me that we don't use use standard metric units. For example, I was reading about the Psyche mission to an asteroid about 2-3 AU away. Why not just say 400 to 500 Gm. Or the nearest star is 4.25 light years away. Why not just say 40 Pm?

Surely one advance in science is the use a consistent measurement system across all, so that are aren't always tied up converting chains to furlongs to potrzebie.

Answer 1

From Kepler to the 1960's, the AU was the best measurement basis available for astronomy. Planetary motions could be expressed precisely in AU, but the relation of the AU to terrestrial units was only known to 4 significant digits at best. It took the development of interplanetary radar to change this.

The AU thus represents hundreds of years of measurements, while the meter has only become useful here within living memory. The parsec is derived from the AU.

Answer 2

In general, people find really enormous numbers hard to fathom, and this is true even for scientists. When using the metric system, it's not so bad hard to understand kilo-, mega-, and occasionally giga-, but beyond that things get murky. While it's common to use kilometers when referring to distances on or near earth, megameters and gigameters never caught on because they're not needed for everyday experiences.

So when referring to enormous distances, astronomers prefer to use more traditional units so they're dealing with smaller numbers. 1 AU = about 1.5e8 km, so it's a convenient unit for measurements within the solar system. For interstellar distances, light-years and parsecs are easier to work with. It's much nicer to say that Alpha Centauri is 4.3 light-years than 40.6 trillion kilometers (or 40.6 petameters).

One of the benefits of the metric system is how easy it is to switch among scales -- going from milimeters to meters is dividing by 1000, which just moves the decimal point. But this doesn't come up much when dealing with astronomical distances (although they do use metric prefixes to scale up to galactic and intergalactic distances, by using kiloparsecs).

Note that this differs between scientific disciplines. Particle physicists are happy referring to giga-electron volts. The differences are likely historical -- the metric system was well established by the time it became necessary to deal with these quantities. But astronomy is a much older science.

Answer 3

Let's take the discussion to the distances of objects which astronomers are currently studying. Alpha Centauri at 40 Pm doesn't get a lot of attention anymore. The Galatic center is at 220 Em and the closest galaxy, Andromeda, is at 21 Zm. The Coma Supercluster of galaxies is around 2.8 Ym. Anything further out and you have to start using redshift to measure distance.

Don't know about you, but I had to look those last 3 prefixes up. You can find them in Haliday & Resnick, but not in Cutnell & Johnson (both popular first year physics textbooks) which tops out at tera. I don't carry the conversion for peta around in my head like I can for the smaller ones.

It's all a question of scale and the numbers that we can keep in our head. This is why we use kiloparsecs within the galaxy and megaparsecs for extra-galatic objects.

Answer 4

In this specific case, AU and parsec are directly relevant to how astronomers measure the distance to objects using parallax. You can research this yourself, but astronomers measure the position in the sky of an object, and then again six months later, when the Earth is on the opposite side of the Sun, and use the equation:

$$ d=~\frac{1~\rm{AU}}{\theta}$$

where $\theta$ is the angle between the two observed positions, and $d$ is the distance from Earth. If $d$ is measured in parsecs, the equation is simply

$$ d=~\frac{1}{\theta}$$

When astronomers write papers for other astronomers to read and analyze, it is more sensible to publish distances in pc, kpc, Mpc, etc – rather than pointlessly converting to some number like 120 trillion meters, only to have the reader convert back to a unit that he actually uses.

More generally, I think in speaking and writing about astronomy, we should actually lean more heavily into relative distances. Saying "the Moon is 384,000 km from Earth" conveys essentially no information to the reader, other than "really far away." It is comparing the positions of celestial bodies to the distance between my hometown and the neighboring town. But if I say, "the Moon is about 29 Earth diameters away" that gives you a pretty clear picture of the relative positions, and even the ability to build your own scale model of the Earth-Moon system.

Likewise, "the Voyager 1 probe is 24.3 billion km from the Sun" vs. "Voyager 1 is 164 AU from the Sun"..."Voyager 1 is 17 times further from the Sun than Saturn."

Astronomical distances are so beyond human imagination that the only way we can form any meaningful picture at each scale is in relative terms to the next-lowest scale. And even this can barely suffice in cases. E.g. The Milky Way diameter is about 100 million times the solar system diameter. That means if the solar system (to Neptune's orbit) were the size of a baseball, the Milky Way would be about the size of the Earth. No one can really picture the whole planet Earth or its size as a matter of experience – so even this direct ratio "object in question to object at next lowest scale" does not give us much of an intuition. But it is certainly better than comparing the size of the Milky Way to the size of a meter stick.

Answer 5

It depends on the context and scientific branch. Astronomy and related fields deal with distances so vast, that measuring them in run-of-the-mill Earth units, such as kilometers or miles, would result in an extremely large number! Likewise, the masses of celestial bodies are too big for the various types of ton used in contexts other than astronomy. Thus astronomy (and the other astro fields) has its own system for measuring the insanely immense sizes and distances involved. In other sciences, like quantum physics, sizes, distances, times, etc are so small, that even the tiniest of regular units of measurement are far too big to be useful without having long chains of decimals or fractions.

Answer 6

I think it is some kind of mental inertia of the field.

Expressing distances in km (like 9.461e+12 km) is equally wrong, would be better written 9.461 Pm (peta meter). Maybe people are not used to handle mental comparisons in large SI prefixes, for example 1000 light years would be 9.4Em and a million light years would be 9.4Zm. Likely people need the same unit (light year) to perform comparisons, would be "hard" to operate with Z/E/P multiplier prefixes.

When I've learned physics I was told that a major source of mistakes is unit conversion, sometimes we confuse 1Km for a meter, second for hour, inch for a cm, and so on, especially when working with constants, like a density, the gravitational acceleration or submultiples of units, such as yards, feet and inches. SI provides a better handling of this class of mistakes.

And it didn't happen in school only, happens to the professionals too: https://www.latimes.com/archives/la-xpm-1999-oct-01-mn-17288-story.html

Computer science took the SI prefixes and transformed them in the power of two. A KB is 1024B, a MB is 1024*1024B and so on. That was caused by practical reasons, when building a memory chip it is convenient to align its size to a power of two. (1024=2^10). Later, the hard drive manufacturers (these are "linear" magnetic devices, they don't have the need to align to power of two), repurposed the old SI prefixes to advertise their capacities. In hard drives terms, 1MB is one million bytes. However, computing people don't have a problem to convert between EB, PB, TB, GB, MB and KB, I've never heard somebody saying "my database is five million MB", but rather they will say "5 TB". That's because we are equally used to all prefixes, especially in the big data field (some maybe don't use EB and PB so often).

Answer 7

Answer: Because those scientists are humans.

One of the reasons for the plethora of units is that human brains handle small and large numbers differently. There is an extensive discussion of this in “Wild Minds” by Marc Hauser. I’ll summarize: all animals, including humans, can intuitively count up to about 5. Beyond that, they resort to systematic naming or other formal strategies.

Animals need to count. If 3 lions walk behind a bush but only 2 walk out the other side, the gazelle they are stalking needs to know there is still one lion behind the bush.

If someone drops a few coins (up to 5) you don’t need to count them. You can just “see” there are, for instance, 4. But larger groups (say 12) need a different process. You could name them in a learned sequence (1,2,3,…) or visually divide them into 3 groups of 4. But it would be very difficult to just “see” it as a group of 12.

5 or 6 is the upper boundary of objects most people can “see” without reverting to counting. Dice have six sides, but the dots are in patterns to aid recognition. 5 is radially symmetric while 6 is bilaterally symmetric. If 5 and 6 were both dots in a circle (like the EU flag) identification errors would increase.

Same with fractions. Humans are good at estimating fractions down to about 1/5. That’s why measurements with traditional analogue instruments are accepted down to 1/5 of the scale marks (unless a Vernier scale is added).

Traditional units were usually developed so that quantities could be converted numerically between 1/5 and 5. That’s where all those weird units like chains, stones and gills came from. A standard British bar shot is 1/5 of a gill (1 oz) and a gallon in 4 quarts. It is easy to picture 5 shot glasses or 4 quart bottles.

Metric units usually use multipliers of 1000 between units. This makes conversions as easy as shifting a decimal point. But it makes it difficult to picture the resulting quantities. I can’t picture 183cm. But I can picture 6 feet. The person described may weigh 168 lb, which I can’t picture, but 12 stone is a bit easier.

I have nothing against the metric system. It is very rational and excels for many of the purposes I use it for. I cook in metric, build my house in imperial fractions and use machine tools in decimal inches. And I describe solar system distances in AU.

No matter how far we go in space, we are still human and still limited by our evolved mental abilities. People will continue to use unit systems which suit themselves as well as the application.