<p>I would suggest it also makes the material more reachable for the human mind.</p>

<p>I just can't work with insanely large or small numbers.  They convey no meaning.</p>

<p>But 1 AU is easy, even if I don;t know exactly what that is in meters, I know what it <em>means</em> and it is a convenient scale for the mind.</p>

<p>Likewise when we talk about stellar distances, what use is the distance in meters (or AU) ?  It makes more sense to work with light years.  Again most people know what that means even if they don't know exactly what it is in meters.</p>

<p>And when we go cosmic you're also talking about colossal <em>times</em> in the past, so light years do convey a double meaning here.  If I told you the distance in meters, that doesn't instantly tell you how far back in time it is as well.</p>

<p>So I think it's a matter on convenience and comprehension.</p>

CC BY-SA 3.0
2017-03-20T20:58:00.687Z
I want to give you 2 +'s. One for pointing out that it is just more intuitive, to use different units at different scales. Another for pointing out that at large enough scales, it helps to talk about time and distance together.
Shane
CC BY-SA 3.0
2017-03-20T21:57:19.447Z
What about bytes? Nobody seems to have a problem using bytes for extremely large numbers, wether it's KB, MB, GB, TB, PB, etc. Nobody thinks these units are unintuitive or we need a completely different unit once the size exceeds some limit. I'm not sure why this would be different concerning the meter and large measurements.
Arne
CC BY-SA 3.0
2017-03-20T23:21:02.333Z
My view is that KB, MB, TB and so on are not really understood at all by most people.  What's a byte ?  What's a TB ?  For the majority they're little more than marketing labels.  I think the only people who understand them are professionals who have to.  And to a computer type (guilty) those measurements are pretty straightforward.  YMMV.
StephenG - Help Ukraine
CC BY-SA 3.0
2017-03-20T23:45:06.410Z
@Arne: As a computer science major, I would like to point out that we (computer scientists) use a non-SI number of bytes in talking about memory. KB, MB, GB, TB, PB, etc. are not SI units. For example, 1 MB = 1024 KB, not 1000 like it would in an SI system. We use base 2, not base 10.
sharur
CC BY-SA 3.0
2017-03-21T04:57:08.823Z
If it requires some time for the light to travel a distance, and that no information can arrive sooner, then the distant event *is* contemporary so far as your frame of reference is concerned.
can-ned_food
CC BY-SA 3.0
2017-03-21T05:48:03.727Z
The difference is the use of the prefix which gives you the facility of putting a relatively small number in front of the unit. Which comparison is easier?956724 B with 9484536 B or 0.956724 MB with 9.484536 MB. In effect you are renaming the unit.
Farcher
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2017-03-21T13:09:53.443Z
@sharur As a computer science major, you should know better, and use KiB, MiB, etc. Base 2 is outdated.
pipe
CC BY-SA 3.0
2017-03-21T15:31:45.060Z
A major problem with binary prefixes is that the meaning of "0.99 MiB" is not obvious.  0.99 MiB = 1013.76 KiB?  It throws the whole point/benefit of the decimal system out of the window.
gerrit
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2017-03-21T15:48:47.050Z
@gerrit It's so simple: 0.111110101 MiB is exactly 111110101 KiB.....
Thorsten S.
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2017-03-21T15:52:13.980Z
@pipe KiB, MiB, ... are *by definition* base-2. KB, MB, ... are ambiguous, and can use either base-2 or base-10 in common usage.
user
CC BY-SA 3.0
2017-03-21T16:41:39.077Z
@MichaelKjörling Exactly. The quote "We use base 2" is not true anymore.
pipe
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2017-03-21T16:49:54.853Z
I suggest moving the side discussion about computer units to chat (or the appropriate forum) as we're loosing focus on the question.
StephenG - Help Ukraine
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2017-03-21T23:47:40.580Z
@pipe: On the contrary, base 2 is built into the hardware at its most basic level.  What's borderline fraud are the marketers who use powers of 10 to exaggerate the size of their memory.
jamesqf
CC BY-SA 3.0
2017-03-26T14:15:36.743Z
Some mass market portable electronic devices are marketed as being capable of holding X number of songs or HD videos, despite the obvious difficulty of deciding what the actual size of a song or video is.
Lie Ryan
CC BY-SA 3.0
2017-03-20T19:07:19.887Z
2017-03-20T19:07:19.887Z
StephenG - Help Ukraine
<p>Another not yet mentioned reason:</p>

<p><strong>There were no usable SI prefixes for such distances.</strong></p>

<p>If you want to use an unit, you need something which allows to express
a specific quantity without too many leading or trailing zeroes.
I do not express human height as 1 670 000 µm or the size of a bacteria as 0.000 02 m.</p>

<p>If you look up the <a href="https://en.wikipedia.org/wiki/Metric_prefix" rel="noreferrer">table of prefixes</a> you see that giga and tera
was <em>defined</em> the first time 1960. But definition does not include
usage and those definitions were exactly as exotic as <em>octillion</em>; sure
it exists as definition, but noone uses it or knows of its existence.
During academic studies in physics in the 90s (!) <strike>it still was not widely known, 30 years after introduction. Still many scientists do not use
giga- or tera- at all.</strike> Hint by gerrit: Physicists used frequencies with the giga-/tera- prefix, I forgot that.</p>

<p>1 AU is then 150 gigameter or 0.15 terameter. If you are using light years,
1 light year is already 9500 terameter which is not a convenient unit. Thirty years later they finally introduced some usable metric prefixes, but I still
have to find someone who uses exa-, peta-, yotta- or zetta-.</p>

CC BY-SA 4.0
2018-06-18T12:43:23.297Z
Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/79023/discussion-on-answer-by-thorsten-s-why-dont-astronomers-use-meters-to-measure).
called2voyage
CC BY-SA 3.0
2017-03-21T12:51:46.003Z
2017-03-21T15:34:24.463Z
2017-03-21T15:34:24.463Z
Thorsten S.
<p>Along with the other answers, there is one other reason, specifically when measuring the distances to other galaxies.</p>

<p>When stating the distance to other galaxies, Astronomers rarely ever state the distance in any unit of length, they tend to use redshifts (<em>z</em>).  This unit is <strong>not</strong> actually a unit of length (it is a dimensionless ratio of wavelengths), nor does it linearly convert to a distance (<em>z=2</em> is <strong>not</strong> twice as far as <em>z=1</em>), nor is there an excepted conversion between redshift and distance (it depends on what model of the universe you assume).</p>

<p>Redshift is used because it can be very accurately measured.  There are features in a star or a galaxies spectra that we know the exact wavelength that they are emitted at and so the redshift can be calculated exactly by:</p>

<p>$$
z=\frac{\lambda_{obs}}{\lambda_{em}}-1
$$</p>

<p>This is an observed, exact (within experimental error) property.  Converting this to a distance is confusing: are you talking about the distance the object is away from us <em>instantaneously now</em>, or <em>instantaneously when the photon that you see was emitted</em>, or <em>the distance the photon you see travelled</em>?  Do you wish to take into consideration local movement as well as Hubble (universe) expansion?  Add on to this the shape of the universe, the rate of expansion of the universe, the rate of change of the expansion of the universe (dark energy/Hubble constants/other effects), and you see that any conversion to an actual distance is problematic and would require that you define exactly what type of conversion and with what assumptions.  It is easier to stay with the well-defined easy-to-measure redshift.</p>

<p>A good (degree-level) work that summarises all the different types of cosmological distances and their calculations is <a href="https://arxiv.org/abs/astro-ph/9905116" rel="noreferrer">Hogg 2000</a>.</p>

CC BY-SA 3.0
2017-10-20T13:58:19.637Z
Jonathan: in Hogg Introduction, Is it correct the all distance are measured along a null radial line? Gravitational lensing came to my mind... In the sense that obviously a photon terminate at me as observer, but I would expect (in principle, not in the absolute sense... Difference can be negligible) that it does after having "curved". I hope is clear what I mean.
Alchimista
CC BY-SA 3.0
2017-03-23T11:07:21.180Z
2017-03-23T11:07:21.180Z
Jonathan Twite
<p>Perhaps one needs to go back in time and think about why the cubit (length of forearm), league (distance walked in one hour), foot, (metre - one ten-millionth of a quadrant of the Earth?? and so perhaps should not be i this list) etc were chosen as the units of distance?<br>
They were easily understood and reproducible  whilst at the same time being of a scale comparable with distances to be measured.<br>
So in the modern world people have chosen further units of distance which initially had those characteristics.</p>

<p>Once these new units gain favour and papers, textbooks etc are written it is difficult to get rid of them and some would say - "Why bother?".  </p>

CC BY-SA 3.0
2017-03-21T06:05:47.050Z
2017-03-21T09:53:26.943Z
2017-03-21T09:53:26.943Z
Farcher
<p>Several excellent answers have already been given. But no one has talked about logarithmic perception. (<a href="https://en.wikipedia.org/wiki/Weber%E2%80%93Fechner_law" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Weber%E2%80%93Fechner_law</a>)</p>
<p>We perceive everything logarithmic-ally. For humans, the difference between <span class="math-container">$10 metres$</span> and <span class="math-container">$100 metres$</span> is the same as between <span class="math-container">$100 metres$</span> and <span class="math-container">$1 km$</span>.</p>
<blockquote>
<p><a href="https://i.sstatic.net/S5zcj.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/S5zcj.png" alt="Weber-Fechner Law" /></a></p>
<p>An illustration of the Weber–Fechner law. On each side, the lower square contains 10 more dots than the upper one. However the perception is different: On the left side, the difference between upper and lower square is clearly visible. On the right side, both squares look almost the same.</p>
</blockquote>
<p>Hence it is much better to measure distance on astronomical scales in <strong>parsecs</strong> than <strong>metres</strong> because humans understand the difference between <span class="math-container">$1$</span> and <span class="math-container">$10$</span> parsecs better than they would do if the same data was presented in meters.</p>

CC BY-SA 3.0
2017-07-24T09:20:38.373Z
"humans understand the difference between 1 and 10 parsecs better than they would do if the same data was presented in meters." Just add one of the SI prefixes for meters and you end up with the same numerical situation. This doesn't really explain why parsecs and not petameters (Pm).
NoDataDumpNoContribution
CC BY-SA 3.0
2017-07-24T16:10:32.250Z
You could have named parsecs as _petameters_. We just decided that parsec sounded better.
Agile_Eagle
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2017-07-24T16:56:45.170Z
also parsec is convenient since its definition makes it very easy to compute distance using parallax
Agile_Eagle
CC BY-SA 3.0
2017-07-24T19:27:22.173Z
I fully agree, it was very convenient. I think that in the end it's mostly a matter of convention.
NoDataDumpNoContribution
CC BY-SA 3.0
2017-07-24T05:54:39.303Z
2017-07-24T05:54:39.303Z
2020-06-17T09:47:40.653Z
Agile_Eagle
<p>I do not know how it is in your country, but here in Russia, astronomical articles and news very often report astronomical distances in kilometers, million kilometers, billion kilometers, trillion kilometers etc. It is just we do not use units like gigameters, petameters and the like, but kilometer is the standard unit in astronomy.</p>

CC BY-SA 3.0
2017-03-25T18:41:16.680Z
I think you're talking about articles in popular publications, but not professional astronomical journals.
Walter
CC BY-SA 3.0
2017-03-22T03:27:50.873Z
2017-03-23T12:16:02.280Z
2017-03-23T12:16:02.280Z
Anixx
<p><strong>Astronomers don't and cannot <em>measure</em> distances.</strong> Distances are merely inferred from what actually has been measured, such as an angle, a relative luminosity, a time period, etc.. Most astronomical distance determinations ultimately hinge on the Earth-Sun distance (astronomical unit), which therefore is of fundamental importance (and only in modern time is known with good accuracy). For nearby stars, the parallax angle is directly related to the distance, but the distance inferred from that is not a proper measured  distance: its uncertainty is not normally distributed (think about a negative parallax measurement).</p>

<p>Astronomers know, of course, how many meters a parsec is, and know that using meters for galactic distances is only confusing, because you have to make sure you get the correct number of 0000 all the time (or the correct power of ten).</p>

<p>Finally, unlike particle physics, astronomy as a science predates the meter system, at least its wider use. Changing from a well working system to something else only for the sake of conformity with SI, but for the price of inconvenience and confusion seems a stupid idea.</p>

CC BY-SA 3.0
2017-07-24T09:17:51.900Z
"Distances are merely inferred from what actually has been measured..." Isn't this always like this? Observations are rarely direct and often you have to infer the value you are interested in in some way or another. This doesn't make it a less valid measurement. It's simply wrong to state that you cannot measure distances in astronomy.
NoDataDumpNoContribution
CC BY-SA 3.0
2017-03-25T18:39:25.687Z
2017-03-25T18:46:00.967Z
2017-03-25T18:46:00.967Z
Walter
<p>Units like metres are simply too small to be used when measuring distances on an astronomical scale. While one could, in theory, use metres in conjunction with scientific notation, it is unnecessarily difficult. One Astronomical Unit is the distance between the Earth and the Sun, this acts as a sort of a cosmic metre stick.</p>

CC BY-SA 3.0
2017-03-22T13:58:08.443Z
Except the distance between the Sun and the Earth keeps changing, so the AU needed to be defined in some invariant units anyway...
user
CC BY-SA 3.0
2017-03-27T12:43:57.553Z
The AU is the semi-major axis, which is pretty close to invariant.
userLTK
CC BY-SA 3.0
2017-07-24T09:24:50.950Z
"Units like metres are simply too small..." Then use a prefix to make them larger like for example a petameter (Pm). I don't see the big disadvantage.
NoDataDumpNoContribution
CC BY-SA 3.0
2017-03-21T16:00:14.827Z
2017-03-21T16:00:14.827Z
Danielwalsh100
<p>It's tough to relate something like a terameter to "real lengths", because of the lack of knowledge of physical objects to compare them to.  Also, because after a while, these units become just "so many more zeros".  So I would suggest the following:</p>

<p>Space Marginal Unit (SMU): 1,000,000 meters, or roughly the distance of from one end of France to the other.  The minimum distance two spacecraft would have to be from each other before they would have to coordinate trajectories or go into docking manuvers.  (Give me a bit of suspension of disbelief here folks.)</p>

<p>Length of Earth Orbit (LEO): 1,000,000,000,000 meters, the distance the Earth travels in one year.  (The distance is actually about 6% less than that, but the LEO is something that can be visualized.)</p>

<p>Kaid: 1,000,000,000,000,000,000 meters.  That's a bit more than the distance from here to the star Alkaid.</p>

<p>The above readily lend themselves to everyday conversation -- if we ever get to a point where we talk about such things everyday!</p>

CC BY-SA 3.0
2017-03-22T13:57:12.317Z
I don't see how this answers the question. Also, LEO is the common abbreviation for *Low Earth Orbit*, which is something very unlike the Earth's orbit around the Sun.
user
CC BY-SA 3.0
2017-07-24T09:27:00.760Z
"It's tough to relate something like a terameter to "real lengths"" Really? For me a parsec is equally tough to relate it to a length that I can feel. My simple view is that some stars and galaxies are just really, really far away. And that 1 terameter is clearly defined and therefore must have a meaning.
NoDataDumpNoContribution
CC BY-SA 3.0
2017-03-21T10:51:24.707Z
2017-03-21T10:51:24.707Z
Jennifer
<p>Because distance is <strong>lumpy</strong>. But bytes, booms, and buzzes vary <strong>smoothly</strong>.</p>

<p>The examples from the question, where metric prefixes became conventional -- terabytes, megatons, gigahertz -- are domains where human experience proceeded continuously across orders of magnitude.</p>

<ul>
<li><p>There were no hard, persistent thresholds in the <a href="https://en.wikipedia.org/wiki/Moore%27s_law" rel="nofollow noreferrer">growth of hard drives, ICs, or cables</a>. Except for a little stickiness at the powers of 2, that progress was continuous.</p></li>
<li><p>Explosions grew gradually over history. There were rare leaps such as nuclear weapons but there are no magic numbers. If every fusion bomb had the same yield then maybe that would have become a scientific unit, but <a href="https://en.wikipedia.org/wiki/List_of_nuclear_weapons_tests#Most_powerful_tests" rel="nofollow noreferrer">they varied all over the place</a>.</p></li>
<li><p>There are few magic frequencies long familiar to humans. Electromagnetic waves have a vivid island at <a href="https://en.wikipedia.org/wiki/Visible_spectrum" rel="nofollow noreferrer">visible light</a>. But even that is smeared across an octave (400-800 TeraHertz) and there are wide oceans of unremarkable uniformity to either side. </p></li>
</ul>

<p>Human acquaintance with <strong>distance</strong> on the other hand proceeded in fits and starts. "<a href="https://vimeo.com/2822787" rel="nofollow noreferrer">We were bounded only by the earth, and the ocean, and the sky,</a>" said <a href="https://youtu.be/YH3c1QZzRK4" rel="nofollow noreferrer">Sagan</a>. Those hard boundaries on human travel persisted for millenia. The stride of an adult is an ancient, narrow, <em>familiar</em> island on the spectrum of distances. </p>

<h1>The distance to the sun was always familiar, and apparently large, long before anyone could measure it.</h1>

<p>So terms for these distances persist, a meter, an Astronomical Unit. Perhaps it's arbitrary that everything longer isn't measured with metric prefixes in AU. But a "lightyear" (63 kilo-AU) does anchor that freakish distance onto two tangibles that could hardly be more familiar, even if their combination is not.</p>

<p><strong>Time</strong> is another lumpy domain for humans, with deep ruts at the extent of a day, a year, a breath. So "gigasecond" (about 32 years) would be nonsense.</p>

<p>For distance and time, no single unit will do.</p>

CC BY-SA 4.0
2019-08-30T20:39:58.200Z
2020-02-18T17:01:42.763Z
2020-02-18T17:01:42.763Z
Bob Stein
<p>The simple answer is: the larger units such as AU or light years are easier for the human brain to remember. And we should avoid putting units with to many zeros trailing after the first few digits, for example:1,000,000,000,000,000,000,000,000,000,000 meters. We could use AU, or for even higher distances, light years. If it was shorter, we cold still use meters but with an exponent.</p>

CC BY-SA 4.0
2018-06-17T10:12:59.530Z
1 AU is about 0.15 Tm, if you use the right prefix you don't have excessive zeroes. The size of a water molecule is 0.275 nm, we don't say 0.000000000275 meters.
Arne
CC BY-SA 4.0
2018-06-14T00:44:59.633Z
2022-11-25T10:39:41.720Z
2022-11-25T10:39:41.720Z
Leo Pan
<p>In addition to the answer provided by @HDE226868, there are historical reasons. Before the advent of using radar ranging to find distances in the solar system, we had to use other clever methods for finding the distance from the Earth to the sun; for example, <A HREF="https://science.nasa.gov/science-news/science-at-nasa/2004/28may_cook/" rel="noreferrer">measuring the transit of Venus across the surface of the sun</A>. These methods are not as super accurate as what is available today, so it makes sense to specify distances, that are all based on measuring parallaxes, in terms of the uncertain, but fixed, Earth-Sun distance. That way, if future measurements change the conversion value from AU to meters, you don't have to change as many papers and textbooks.</p>

<p>Not to mention that such calibration uncertainties introduce correlated errors into an analysis that aren't defeatable using large sample sizes.</p>

<p>I can't speak authoritatively on the actual history, but solar system measurements were all initially done in terms of the Earth/sun distance. For example, a little geometry shows that it's pretty straightforward to back out the size of Venus's and Mercury's orbit in AU from their maximum solar elongation. I don't know how they worked out the orbital radii of Mars, etc, but they were almost certainly done in AU long before the AU was known, and all of that before the MKS system existed, let alone became standardized. </p>

<p>For stars, the base of what is known as the "cosmological distance ladder" (that is "all distance measures" in astronomy) rests on measuring the parallax angle:
<span class="math-container">$$\tan \pi_{\mathrm{angle}} = \frac{1 AU}{D}.$$</span>
To measure <span class="math-container">$D$</span> in 'parsecs' is to setup the equation so that the angle being measured in arcseconds fits the small angle approximation. That is:
<span class="math-container">$$\frac{D}{1\, \mathrm{parsec}} = \frac{\frac{\pi}{180\times60\times60}}{\tan\left(\pi_{\mathrm{angle}} \frac{\pi\, \mathrm{radians}}{180\times60\times60 \, \mathrm{arcsec}}\right)}.$$</span>
In other words, <span class="math-container">$1\operatorname{parsec} = \frac{180\times 3600}{\pi} \operatorname{AU}$</span>.</p>

<p>Astronomers also have a marked preference for the close cousin of mks/SI units, known as <A HREF="https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units" rel="noreferrer">cgs</A>. As far as I can tell, this is due to the influence of spectroscopists who liked the "Gaussian units" part of it for electromagnetism because it set Coulomb's constant to 1, simplifying calculations.</p>

CC BY-SA 3.0
2017-03-20T20:08:46.323Z
I would say that this is the correct answer, whereas the one provided by HDE 226868 is not. In terms of human comprehensibility, measuring e.g. the solar system is AU is no more or less intuitive than measuring it in gigameters (or perhaps terameters; 1 AU ≈ 150 Gm = 0.15 Tm). However, the non-metric units still persist due to historical inertia, and the fact that they were (and sometimes still are) more convenient in cases where some distance can be measured in some particular units more accurately than the length of those units themselves can be measures in meters.
Ilmari Karonen
CC BY-SA 3.0
2017-03-20T22:00:03.497Z
I like this answer. You could extend it by mentioning that the favoured measure of stellar distance is the parsec, since it can be calculated exactly in terms of AU, (648000 AU = \pi parsec)
James K
CC BY-SA 3.0
2017-03-21T13:50:55.903Z
Another historical parallel to this situation comes from chemistry, where there is a strong preference to talk about the "moles" of a substance rather than a certain number of molecules of that substance.  It's not just that the number of moles is less likely to require scientific notation to express;  it's also that for a surprisingly long time (until the early 20th century), chemists didn't actually know how many molecules were in a mole.
Michael Seifert
CC BY-SA 3.0
2017-03-21T13:53:44.347Z
In general, physicists don't like raw numbers. They really like to express quantities as dimensionless numbers that express some property of a system. It makes it easier to reason about things. So, if you are considering a planetary system, working in AU (i.e. expressing distances as a multiple of earth's orbit) is a very reasonable thing to do.
drxzcl
CC BY-SA 3.0
2017-03-21T16:54:52.757Z
Astronomers don't seriously use pi_angle for parallax angle, do they? That seems potentially confusing =).
Chris Chudzicki
CC BY-SA 3.0
2017-03-21T17:21:35.050Z
No, they don't, @ChrisChudzicki, it's usually just $\pi$. They also don't usually use radians, so I added the subscript to disambiguate. See: https://en.wikipedia.org/wiki/List_of_common_astronomy_symbols
Sean Lake
CC BY-SA 3.0
2017-03-21T20:02:27.113Z
Astronomers really like Gaussian units for a number of reasons.  Electric and magnetic fields have the same units!  This makes complete sense since they are the same thing in different frames.  The only constant in Maxwell's equations is the speed of light!
KAI
CC BY-SA 3.0
2017-03-21T22:40:32.667Z
@KAI It only makes sense for electric and magnetic fields to have the same units in unit systems where the speed of light is unitless. The relationship between them, in terms of units, is fundamentally no different than the relationship between momentum and energy, etc. Put another way, the vector potential and the electric potential should differ by a factor of $c$.
Sean Lake
CC BY-SA 3.0
2017-03-27T09:53:15.847Z
Isn't it also a question of accuracy ? I remember school, where we studied why units were important : e.g. in a road distance of 58 km (beware this is a compound unit, thus we can see it as "another unit than meters with the same root and a 1000x factor"), it does not matter if you have +/- 0.05km, but if you say that distance is 58000 meters, the acceptable accuracy would be something like +/- 0.05m. Applying this to AU, it does not matter if there is a +/-50000km error in the measure, it is an order of magnitude. Please correct me if I'm not right.
Benj
CC BY-SA 4.0
2019-03-22T04:54:59.567Z
@Benj You are not right. I don't know how old the conventions are, but the sig-fig conventions and scientific notation both handle specifying astronomical distances in AU and meters equally well. Granted, you save a little writing when discussing solar system objects and nearby stars when using AU and parsecs, respectively, but that's a matter of convenience not a technical necessity. That said, you can't underestimate the value of convenience, especially in the pre-computer world.
Sean Lake
CC BY-SA 4.0
2019-04-22T17:41:01.870Z
@SeanLake that's not incompatible and you're rigth the convenience aspect is a real point
Benj
CC BY-SA 4.0
2017-03-20T17:09:52.617Z
2019-09-03T11:52:07.463Z
2019-09-03T11:52:07.463Z
Sean Lake
<p>In astronomy distances are generally expressed in non-metric units like: light-years, astronomical units (AU), parsecs, etc. Why don't they use meters (or multiples thereof) to measure distances, as these are the SI unit for distance? Since the meter is already used in particle physics to measure the size of atoms, why couldn't it be used in astrophysics to measure the large distances in the Universe? </p>

<p>For example:</p>

<ul>
<li>The ISS orbits about 400 km above Earth.</li>
<li>The diameter of the Sun is 1.39 Gm (gigameters).</li>
<li>The distance to the Andromeda Galaxy is 23 Zm (zettameters).</li>
<li>At its furthest point, Pluto is 5.83 Tm (terameters) from the Sun.</li>
</ul>

<p>Edit: some have answered that meters are too small and therefore not intuitive for measuring large distances, however there are plenty of situations where this is not a problem, for example:</p>

<ul>
<li>Bytes are used for measuring gigantic amounts of data, for example terabytes (1e+12) or petabytes (1e+15)</li>
<li>The energy released by large explosions is usually expressed in megatons, which is based on grams (1e+12)</li>
<li>The SI unit Hertz is often expressed in gigahertz (1e+9) or terahertz (1e+12) for measuring network frequencies or processor clock speeds.</li>
</ul>

<p>If the main reason for not using meters is historical, is it reasonable to expect that SI-unites will become the standard in astronomy, like most of the world switched from native to SI-units for everyday measurements? </p>

CC BY-SA 3.0
2017-03-20T22:04:34.327Z
As you noted, we do. They're just in groups of 1,000 or greater.
BruceWayne
CC BY-SA 3.0
2017-03-20T22:23:16.320Z
Because it's not useful to do so.
eyeballfrog
CC BY-SA 3.0
2017-03-21T03:51:55.933Z
Planetary distances are most often measured in Astronomical Units (AU), not km.  Like light years, the unit tells you something useful about the distance, e.g. 1 AU is the average distance from the Earth to the Sun.
jamesqf
CC BY-SA 3.0
2017-03-21T06:35:23.310Z
What do you think an Angstrom or a Fermi are? Or a barn? Physicists don't always specify stuff in SI either and for the same reason.
ProfRob
CC BY-SA 3.0
2017-03-21T08:33:23.143Z
For the same reason that you buy rice in KG, not by the grain.
dotancohen
CC BY-SA 3.0
2017-03-21T11:45:18.293Z
Because you want units to relate to objects being measured. If I told you I'm $1.13*10^{35}$ Plank lengths tall, would it help you to picture how tall I am?
Dmitry Grigoryev
CC BY-SA 3.0
2017-03-21T12:23:28.063Z
@DmitryGrigoryev: that depends on how common it is to use those units for that kind of measure, so it is a matter of tradition. I grew up with the metric system, and to this day it is not very helpful if they tell me that someone is 5'7'' tall;  but I can assure you that it is exactly the opposite for someone who grew up in America.
Martin Argerami
CC BY-SA 3.0
2017-03-21T12:29:46.193Z
@MartinArgerami True, but if someone tells me they are 57 feet tall, I'll spot a mistake right away (and I think an American won't believe me if I tell them I'm 18 meters tall). With Plank lengths, even a mistake by an order of magnitude may not be obvious.
Dmitry Grigoryev
CC BY-SA 3.0
2017-03-22T11:07:51.873Z
I actually know a guy who likes to measure length in attoparsecs.
JohnEye
CC BY-SA 3.0
2017-03-22T13:42:02.347Z
@JohnEye I think we all know That Guy.
chepner
CC BY-SA 3.0
2017-03-24T11:33:13.133Z
Astrophysicists never use light-years, journalists use light-years because they think it is a sciency-concept that the public can understand.  Amusingly if anyone actually understands the *distance* of a light-year, they would have no problem with the *distance* of a parsec.
Jonathan Twite
CC BY-SA 3.0
2017-03-26T18:26:06.467Z
I my thesis, I used centimeters. I did calculations of H2 clouds 10^18 cm in size.
Christine
CC BY-SA 3.0
2017-03-27T03:38:29.907Z
@dotancohen Your "buy rice in KG" is a good example of what the question is asking: we don't mind lugging around an extra factor of 1000 there, even though the metric unit is inconvenient (we use kilogram rather than the metric unit of gram). If we can speak in kilograms and not think it inconvenient, why not speak in gigametres or petametres?
ShreevatsaR
CC BY-SA 3.0
2017-03-27T05:29:32.447Z
@ShreevatsaR: We buy rice by the **kilogram**, not the **kilograin**. I get what you are saying, but the amount of grains we eat don't really, matter. What matters is the mass of food that we eat. By measuring rice in kilograms and also potatoes in kilograms, we can compare portions. Similarly, the AU and lightyear give us a better way to compare distances than do the SI prefixes with the meter.
dotancohen
CC BY-SA 3.0
2017-03-27T05:35:18.657Z
@dotancohen I didn't mention grains in my comment. I only pointed out that we _do_ indeed buy rice by the **kilo**gram, not gram. If we can measure different quantities in kilograms (an SI prefix with the gram), we can also measure different distances in (say) terametres (and compare them that way).
ShreevatsaR
CC BY-SA 3.0
2017-07-24T09:31:06.110Z
@dotancohen "Similarly, the AU and lightyear give us a better way to compare distances than do the SI prefixes with the meter." I fail to see how this is better? But I agree that kilogram is better than kilograin. I would conclude that then petameters must be better than some obscure other unit, because it only matters that it's a length (a really long length but way beyond our imagination anyway). AU and pc are there because they were first and have historical relevance. In the end it's all convention. But I wouldn't bet in 100 years people will still use it.
NoDataDumpNoContribution
CC BY-SA 3.0
2017-09-22T13:14:33.057Z
"Megatons" is actually a good example of a non-SI unit. A megaton is not a unit of mass, it's a unit of energy = 4.184 gigajoules. (It's indirectly based on energy released from the explosion of a "TNT-equivalent" mass, but as an energy unit it's clearly not a standard, power-of-ten SI unit.)
Peter Erwin
CC BY-SA 4.0
2018-06-14T08:24:19.963Z
My favourite are Light Years and AU... Rather than raw maths, most humans can create a clearer mind-map and logical framework with comparative images. originally using feet, steps, hands, counting 20's because of fingers... It's natural for humans to use earth-sun and light-year to logically interpret space distances. AU is especially easy. Parsecs are simple images too when you imagine AU's. They are only weird when you are fresh out of prelim science studies.. Syllabels with TERS dont lend themselves to stars conversations, it's also preferential linguistics, same reason how french say 97.
bandybabboon
CC BY-SA 4.0
2019-09-03T17:11:04.167Z
IIRC, AU are useful because for the longest time we weren't sure how exactly far away the sun is, but we did know distances of other planets compared to the earth thanks to Kepler's third law. And the math of Kepler's law becomes easier when you reduce the earth's distance to 1.
pboss3010
CC BY-SA 4.0
2019-09-03T19:12:19.863Z
1 TM (terameter) would be $\approx$ 6 AU. 1 EM (exameter) would by $\approx$ 11 ly. 1 YM (yottameter) would be 11million light years, still a thousandth of the visible Universe, but the scale ends here.
peterh
CC BY-SA 4.0
2019-09-03T19:21:09.973Z
The radius of the visible Universe would be $\approx$ 1200 YM. I think it wouldn't be so bad. However, using the Planck units is not possible, because they all depend on the gravitational constant, and we know it only with $\approx$ 5 digits precision. The SI meter depends only the SI second and on the speed of light, both are known very precisely. Maybe if once we will be able to know also G with at least 10-12 digits precision, a new scale based on purely the Planck units will become useful.
peterh
<distances><units>
Why don't astronomers use meters to measure astronomical distances?
Arne
# Why don't astronomers use meters to measure astronomical distances?

(Asked by: username_0 on 2017-03-20)

In astronomy distances are generally expressed in non-metric units like: light-years, astronomical units (AU), parsecs, etc. Why don't they use meters (or multiples thereof) to measure distances, as these are the SI unit for distance? Since the meter is already used in particle physics to measure the size of atoms, why couldn't it be used in astrophysics to measure the large distances in the Universe?

For example:

* The ISS orbits about 400 km above Earth.
* The diameter of the Sun is 1.39 Gm (gigameters).
* The distance to the Andromeda Galaxy is 23 Zm (zettameters).
* At its furthest point, Pluto is 5.83 Tm (terameters) from the Sun.

Edit: some have answered that meters are too small and therefore not intuitive for measuring large distances, however there are plenty of situations where this is not a problem, for example:

* Bytes are used for measuring gigantic amounts of data, for example terabytes (1e+12) or petabytes (1e+15)
* The energy released by large explosions is usually expressed in megatons, which is based on grams (1e+12)
* The SI unit Hertz is often expressed in gigahertz (1e+9) or terahertz (1e+12) for measuring network frequencies or processor clock speeds.

If the main reason for not using meters is historical, is it reasonable to expect that SI-unites will become the standard in astronomy, like most of the world switched from native to SI-units for everyday measurements?

**Tags:** distances, units

> **Comments:**
> **username_1**: As you noted, we do. They're just in groups of 1,000 or greater.  
> **username_2**: Because it's not useful to do so.  
> **username_3**: Planetary distances are most often measured in Astronomical Units (AU), not km.  Like light years, the unit tells you something useful about the distance, e.g. 1 AU is the average distance from the Earth to the Sun.  
> **username_4**: What do you think an Angstrom or a Fermi are? Or a barn? Physicists don't always specify stuff in SI either and for the same reason.  
> **username_5**: For the same reason that you buy rice in KG, not by the grain.  
> **username_6**: Because you want units to relate to objects being measured. If I told you I'm $1.13*10^{35}$ Plank lengths tall, would it help you to picture how tall I am?  
> **username_7**: @DmitryGrigoryev: that depends on how common it is to use those units for that kind of measure, so it is a matter of tradition. I grew up with the metric system, and to this day it is not very helpful if they tell me that someone is 5'7'' tall;  but I can assure you that it is exactly the opposite for someone who grew up in America.  
> **username_6**: @MartinArgerami True, but if someone tells me they are 57 feet tall, I'll spot a mistake right away (and I think an American won't believe me if I tell them I'm 18 meters tall). With Plank lengths, even a mistake by an order of magnitude may not be obvious.  
> **username_8**: I actually know a guy who likes to measure length in attoparsecs.  
> **username_9**: @username_26name_8 I think we all know That Guy.  
> **username_10**: Astrophysicists never use light-years, journalists use light-years because they think it is a sciency-concept that the public can understand.  Amusingly if anyone actually understands the *distance* of a light-year, they would have no problem with the *distance* of a parsec.  
> **username_11**: I my thesis, I used centimeters. I did calculations of H2 clouds 10^18 cm in size.  
> **username_12**: @username_26name_5 Your "buy rice in KG" is a good example of what the question is asking: we don't mind lugging around an extra factor of 1000 there, even though the metric unit is inconvenient (we use kilogram rather than the metric unit of gram). If we can speak in kilograms and not think it inconvenient, why not speak in gigametres or petametres?  
> **username_5**: @username_26name_12: We buy rice by the **kilogram**, not the **kilograin**. I get what you are saying, but the amount of grains we eat don't really, matter. What matters is the mass of food that we eat. By measuring rice in kilograms and also potatoes in kilograms, we can compare portions. Similarly, the AU and lightyear give us a better way to compare distances than do the SI prefixes with the meter.  
> **username_12**: @username_26name_5 I didn't mention grains in my comment. I only pointed out that we _do_ indeed buy rice by the **kilo**gram, not gram. If we can measure different quantities in kilograms (an SI prefix with the gram), we can also measure different distances in (say) terametres (and compare them that way).  
> **username_13**: @username_26name_5 "Similarly, the AU and lightyear give us a better way to compare distances than do the SI prefixes with the meter." I fail to see how this is better? But I agree that kilogram is better than kilograin. I would conclude that then petameters must be better than some obscure other unit, because it only matters that it's a length (a really long length but way beyond our imagination anyway). AU and pc are there because they were first and have historical relevance. In the end it's all convention. But I wouldn't bet in 100 years people will still use it.  
> **username_14**: "Megatons" is actually a good example of a non-SI unit. A megaton is not a unit of mass, it's a unit of energy = 4.184 gigajoules. (It's indirectly based on energy released from the explosion of a "TNT-equivalent" mass, but as an energy unit it's clearly not a standard, power-of-ten SI unit.)  
> **username_15**: My favourite are Light Years and AU... Rather than raw maths, most humans can create a clearer mind-map and logical framework with comparative images. originally using feet, steps, hands, counting 20's because of fingers... It's natural for humans to use earth-sun and light-year to logically interpret space distances. AU is especially easy. Parsecs are simple images too when you imagine AU's. They are only weird when you are fresh out of prelim science studies.. Syllabels with TERS dont lend themselves to stars conversations, it's also preferential linguistics, same reason how french say 97.  
> **username_16**: IIRC, AU are useful because for the longest time we weren't sure how exactly far away the sun is, but we did know distances of other planets compared to the earth thanks to Kepler's third law. And the math of Kepler's law becomes easier when you reduce the earth's distance to 1.  
> **username_17**: 1 TM (terameter) would be $\approx$ 6 AU. 1 EM (exameter) would by $\approx$ 11 ly. 1 YM (yottameter) would be 11million light years, still a thousandth of the visible Universe, but the scale ends here.  
> **username_17**: The radius of the visible Universe would be $\approx$ 1200 YM. I think it wouldn't be so bad. However, using the Planck units is not possible, because they all depend on the gravitational constant, and we know it only with $\approx$ 5 digits precision. The SI meter depends only the SI second and on the speed of light, both are known very precisely. Maybe if once we will be able to know also G with at least 10-12 digits precision, a new scale based on purely the Planck units will become useful.  

---

## 12 answers

### Answer by username_38 on 2017-03-20. Score: 89 | Accepted answer

In addition to the answer provided by @HDE226868, there are historical reasons. Before the advent of using radar ranging to find distances in the solar system, we had to use other clever methods for finding the distance from the Earth to the sun; for example, [measuring the transit of Venus across the surface of the sun](https://science.nasa.gov/science-news/science-at-nasa/2004/28may_cook/). These methods are not as super accurate as what is available today, so it makes sense to specify distances, that are all based on measuring parallaxes, in terms of the uncertain, but fixed, Earth-Sun distance. That way, if future measurements change the conversion value from AU to meters, you don't have to change as many papers and textbooks.

Not to mention that such calibration uncertainties introduce correlated errors into an analysis that aren't defeatable using large sample sizes.

I can't speak authoritatively on the actual history, but solar system measurements were all initially done in terms of the Earth/sun distance. For example, a little geometry shows that it's pretty straightforward to back out the size of Venus's and Mercury's orbit in AU from their maximum solar elongation. I don't know how they worked out the orbital radii of Mars, etc, but they were almost certainly done in AU long before the AU was known, and all of that before the MKS system existed, let alone became standardized.

For stars, the base of what is known as the "cosmological distance ladder" (that is "all distance measures" in astronomy) rests on measuring the parallax angle:
$$\tan \pi\_{\mathrm{angle}} = \frac{1 AU}{D}.$$
To measure $D$ in 'parsecs' is to setup the equation so that the angle being measured in arcseconds fits the small angle approximation. That is:
$$\frac{D}{1\, \mathrm{parsec}} = \frac{\frac{\pi}{180\times60\times60}}{\tan\left(\pi\_{\mathrm{angle}} \frac{\pi\, \mathrm{radians}}{180\times60\times60 \, \mathrm{arcsec}}\right)}.$$
In other words, $1\operatorname{parsec} = \frac{180\times 3600}{\pi} \operatorname{AU}$.

Astronomers also have a marked preference for the close cousin of mks/SI units, known as [cgs](https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units). As far as I can tell, this is due to the influence of spectroscopists who liked the "Gaussian units" part of it for electromagnetism because it set Coulomb's constant to 1, simplifying calculations.

> **Comments:**
> **username_39**: I would say that this is the correct answer, whereas the one provided by HDE 226868 is not. In terms of human comprehensibility, measuring e.g. the solar system is AU is no more or less intuitive than measuring it in gigameters (or perhaps terameters; 1 AU ≈ 150 Gm = 0.15 Tm). However, the non-metric units still persist due to historical inertia, and the fact that they were (and sometimes still are) more convenient in cases where some distance can be measured in some particular units more accurately than the length of those units themselves can be measures in meters.  
> **username_40**: I like this answer. You could extend it by mentioning that the favoured measure of stellar distance is the parsec, since it can be calculated exactly in terms of AU, (648000 AU = \pi parsec)  
> **username_41**: Another historical parallel to this situation comes from chemistry, where there is a strong preference to talk about the "moles" of a substance rather than a certain number of molecules of that substance.  It's not just that the number of moles is less likely to require scientific notation to express;  it's also that for a surprisingly long time (until the early 20th century), chemists didn't actually know how many molecules were in a mole.  
> **username_42**: In general, physicists don't like raw numbers. They really like to express quantities as dimensionless numbers that express some property of a system. It makes it easier to reason about things. So, if you are considering a planetary system, working in AU (i.e. expressing distances as a multiple of earth's orbit) is a very reasonable thing to do.  
> **username_43**: Astronomers don't seriously use pi_angle for parallax angle, do they? That seems potentially confusing =).  
> **username_38**: No, they don't, @ChrisChudzicki, it's usually just $\pi$. They also don't usually use radians, so I added the subscript to disambiguate. See: https://en.wikipedia.org/wiki/List_of_common_astronomy_symbols  
> **username_44**: Astronomers really like Gaussian units for a number of reasons.  Electric and magnetic fields have the same units!  This makes complete sense since they are the same thing in different frames.  The only constant in Maxwell's equations is the speed of light!  
> **username_38**: @username_44 It only makes sense for electric and magnetic fields to have the same units in unit systems where the speed of light is unitless. The relationship between them, in terms of units, is fundamentally no different than the relationship between momentum and energy, etc. Put another way, the vector potential and the electric potential should differ by a factor of $c$.  
> **username_45**: Isn't it also a question of accuracy ? I remember school, where we studied why units were important : e.g. in a road distance of 58 km (beware this is a compound unit, thus we can see it as "another unit than meters with the same root and a 1000x factor"), it does not matter if you have +/- 0.05km, but if you say that distance is 58000 meters, the acceptable accuracy would be something like +/- 0.05m. Applying this to AU, it does not matter if there is a +/-50000km error in the measure, it is an order of magnitude. Please correct me if I'm not right.  
> **username_38**: @username_45 You are not right. I don't know how old the conventions are, but the sig-fig conventions and scientific notation both handle specifying astronomical distances in AU and meters equally well. Granted, you save a little writing when discussing solar system objects and nearby stars when using AU and parsecs, respectively, but that's a matter of convenience not a technical necessity. That said, you can't underestimate the value of convenience, especially in the pre-computer world.  
> **username_45**: @SeanLake that's not incompatible and you're rigth the convenience aspect is a real point  

---

### Answer by username_18 on 2017-03-20. Score: 27

I would suggest it also makes the material more reachable for the human mind.

I just can't work with insanely large or small numbers. They convey no meaning.

But 1 AU is easy, even if I don;t know exactly what that is in meters, I know what it *means* and it is a convenient scale for the mind.

Likewise when we talk about stellar distances, what use is the distance in meters (or AU) ? It makes more sense to work with light years. Again most people know what that means even if they don't know exactly what it is in meters.

And when we go cosmic you're also talking about colossal *times* in the past, so light years do convey a double meaning here. If I told you the distance in meters, that doesn't instantly tell you how far back in time it is as well.

So I think it's a matter on convenience and comprehension.

> **Comments:**
> **username_19**: I want to give you 2 +'s. One for pointing out that it is just more intuitive, to use different units at different scales. Another for pointing out that at large enough scales, it helps to talk about time and distance together.  
> **username_0**: What about bytes? Nobody seems to have a problem using bytes for extremely large numbers, wether it's KB, MB, GB, TB, PB, etc. Nobody thinks these units are unintuitive or we need a completely different unit once the size exceeds some limit. I'm not sure why this would be different concerning the meter and large measurements.  
> **username_18**: My view is that KB, MB, TB and so on are not really understood at all by most people.  What's a byte ?  What's a TB ?  For the majority they're little more than marketing labels.  I think the only people who understand them are professionals who have to.  And to a computer type (guilty) those measurements are pretty straightforward.  YMMV.  
> **username_20**: @username_26name_0: As a computer science major, I would like to point out that we (computer scientists) use a non-SI number of bytes in talking about memory. KB, MB, GB, TB, PB, etc. are not SI units. For example, 1 MB = 1024 KB, not 1000 like it would in an SI system. We use base 2, not base 10.  
> **username_21**: If it requires some time for the light to travel a distance, and that no information can arrive sooner, then the distant event *is* contemporary so far as your frame of reference is concerned.  
> **username_22**: The difference is the use of the prefix which gives you the facility of putting a relatively small number in front of the unit. Which comparison is easier?956724 B with 9484536 B or 0.956724 MB with 9.484536 MB. In effect you are renaming the unit.  
> **username_23**: @username_26name_20 As a computer science major, you should know better, and use KiB, MiB, etc. Base 2 is outdated.  
> **username_24**: A major problem with binary prefixes is that the meaning of "0.99 MiB" is not obvious.  0.99 MiB = 1013.76 KiB?  It throws the whole point/benefit of the decimal system out of the window.  
> **username_25**: @username_26name_24 It's so simple: 0.111110101 MiB is exactly 111110101 KiB.....  
> **username_26**: @username_26name_23 KiB, MiB, ... are *by definition* base-2. KB, MB, ... are ambiguous, and can use either base-2 or base-10 in common usage.  
> **username_23**: @MichaelKjörling Exactly. The quote "We use base 2" is not true anymore.  
> **username_18**: I suggest moving the side discussion about computer units to chat (or the appropriate forum) as we're loosing focus on the question.  
> **username_3**: @username_26name_23: On the contrary, base 2 is built into the hardware at its most basic level.  What's borderline fraud are the marketers who use powers of 10 to exaggerate the size of their memory.  
> **username_27**: Some mass market portable electronic devices are marketed as being capable of holding X number of songs or HD videos, despite the obvious difficulty of deciding what the actual size of a song or video is.  

---

### Answer by username_10 on 2017-03-23. Score: 11

Along with the other answers, there is one other reason, specifically when measuring the distances to other galaxies.

When stating the distance to other galaxies, Astronomers rarely ever state the distance in any unit of length, they tend to use redshifts (*z*). This unit is **not** actually a unit of length (it is a dimensionless ratio of wavelengths), nor does it linearly convert to a distance (*z=2* is **not** twice as far as *z=1*), nor is there an excepted conversion between redshift and distance (it depends on what model of the universe you assume).

Redshift is used because it can be very accurately measured. There are features in a star or a galaxies spectra that we know the exact wavelength that they are emitted at and so the redshift can be calculated exactly by:

$$
z=\frac{\lambda\_{obs}}{\lambda\_{em}}-1
$$

This is an observed, exact (within experimental error) property. Converting this to a distance is confusing: are you talking about the distance the object is away from us *instantaneously now*, or *instantaneously when the photon that you see was emitted*, or *the distance the photon you see travelled*? Do you wish to take into consideration local movement as well as Hubble (universe) expansion? Add on to this the shape of the universe, the rate of expansion of the universe, the rate of change of the expansion of the universe (dark energy/Hubble constants/other effects), and you see that any conversion to an actual distance is problematic and would require that you define exactly what type of conversion and with what assumptions. It is easier to stay with the well-defined easy-to-measure redshift.

A good (degree-level) work that summarises all the different types of cosmological distances and their calculations is [Hogg 2000](https://arxiv.org/abs/astro-ph/9905116).

> **Comments:**
> **username_29**: Jonathan: in Hogg Introduction, Is it correct the all distance are measured along a null radial line? Gravitational lensing came to my mind... In the sense that obviously a photon terminate at me as observer, but I would expect (in principle, not in the absolute sense... Difference can be negligible) that it does after having "curved". I hope is clear what I mean.  

---

### Answer by username_25 on 2017-03-21. Score: 9

Another not yet mentioned reason:

**There were no usable SI prefixes for such distances.**

If you want to use an unit, you need something which allows to express
a specific quantity without too many leading or trailing zeroes.
I do not express human height as 1 670 000 µm or the size of a bacteria as 0.000 02 m.

If you look up the [table of prefixes](https://en.wikipedia.org/wiki/Metric_prefix) you see that giga and tera
was *defined* the first time 1960. But definition does not include
usage and those definitions were exactly as exotic as *octillion*; sure
it exists as definition, but noone uses it or knows of its existence.
During academic studies in physics in the 90s (!) it still was not widely known, 30 years after introduction. Still many scientists do not use
giga- or tera- at all. Hint by username_26name_24: Physicists used frequencies with the giga-/tera- prefix, I forgot that.

1 AU is then 150 gigameter or 0.15 terameter. If you are using light years,
1 light year is already 9500 terameter which is not a convenient unit. Thirty years later they finally introduced some usable metric prefixes, but I still
have to find someone who uses exa-, peta-, yotta- or zetta-.

> **Comments:**
> **username_28**: Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/79023/discussion-on-answer-by-thorsten-s-why-dont-astronomers-use-meters-to-measure).  

---

### Answer by username_22 on 2017-03-21. Score: 6

Perhaps one needs to go back in time and think about why the cubit (length of forearm), league (distance walked in one hour), foot, (metre - one ten-millionth of a quadrant of the Earth?? and so perhaps should not be i this list) etc were chosen as the units of distance?  
They were easily understood and reproducible whilst at the same time being of a scale comparable with distances to be measured.  
So in the modern world people have chosen further units of distance which initially had those characteristics.

Once these new units gain favour and papers, textbooks etc are written it is difficult to get rid of them and some would say - "Why bother?".


---

### Answer by username_30 on 2017-07-24. Score: 5

Several excellent answers have already been given. But no one has talked about logarithmic perception. (<https://en.wikipedia.org/wiki/Weber%E2%80%93Fechner_law>)

We perceive everything logarithmic-ally. For humans, the difference between $10 metres$ and $100 metres$ is the same as between $100 metres$ and $1 km$.

> [![Weber-Fechner Law](https://i.sstatic.net/S5zcj.png)](https://i.sstatic.net/S5zcj.png)
>
> An illustration of the Weber–Fechner law. On each side, the lower square contains 10 more dots than the upper one. However the perception is different: On the left side, the difference between upper and lower square is clearly visible. On the right side, both squares look almost the same.

Hence it is much better to measure distance on astronomical scales in **parsecs** than **metres** because humans understand the difference between $1$ and $10$ parsecs better than they would do if the same data was presented in meters.

> **Comments:**
> **username_13**: "humans understand the difference between 1 and 10 parsecs better than they would do if the same data was presented in meters." Just add one of the SI prefixes for meters and you end up with the same numerical situation. This doesn't really explain why parsecs and not petameters (Pm).  
> **username_30**: You could have named parsecs as _petameters_. We just decided that parsec sounded better.  
> **username_30**: also parsec is convenient since its definition makes it very easy to compute distance using parallax  
> **username_13**: I fully agree, it was very convenient. I think that in the end it's mostly a matter of convention.  

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### Answer by username_31 on 2017-03-22. Score: 5

I do not know how it is in your country, but here in Russia, astronomical articles and news very often report astronomical distances in kilometers, million kilometers, billion kilometers, trillion kilometers etc. It is just we do not use units like gigameters, petameters and the like, but kilometer is the standard unit in astronomy.

> **Comments:**
> **username_32**: I think you're talking about articles in popular publications, but not professional astronomical journals.  

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### Answer by username_32 on 2017-03-25. Score: 4

**Astronomers don't and cannot *measure* distances.** Distances are merely inferred from what actually has been measured, such as an angle, a relative luminosity, a time period, etc.. Most astronomical distance determinations ultimately hinge on the Earth-Sun distance (astronomical unit), which therefore is of fundamental importance (and only in modern time is known with good accuracy). For nearby stars, the parallax angle is directly related to the distance, but the distance inferred from that is not a proper measured distance: its uncertainty is not normally distributed (think about a negative parallax measurement).

Astronomers know, of course, how many meters a parsec is, and know that using meters for galactic distances is only confusing, because you have to make sure you get the correct number of 0000 all the time (or the correct power of ten).

Finally, unlike particle physics, astronomy as a science predates the meter system, at least its wider use. Changing from a well working system to something else only for the sake of conformity with SI, but for the price of inconvenience and confusion seems a stupid idea.

> **Comments:**
> **username_13**: "Distances are merely inferred from what actually has been measured..." Isn't this always like this? Observations are rarely direct and often you have to infer the value you are interested in in some way or another. This doesn't make it a less valid measurement. It's simply wrong to state that you cannot measure distances in astronomy.  

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### Answer by username_33 on 2017-03-21. Score: 3

Units like metres are simply too small to be used when measuring distances on an astronomical scale. While one could, in theory, use metres in conjunction with scientific notation, it is unnecessarily difficult. One Astronomical Unit is the distance between the Earth and the Sun, this acts as a sort of a cosmic metre stick.

> **Comments:**
> **username_26**: Except the distance between the Sun and the Earth keeps changing, so the AU needed to be defined in some invariant units anyway...  
> **username_34**: The AU is the semi-major axis, which is pretty close to invariant.  
> **username_13**: "Units like metres are simply too small..." Then use a prefix to make them larger like for example a petameter (Pm). I don't see the big disadvantage.  

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### Answer by username_35 on 2017-03-21. Score: 2

It's tough to relate something like a terameter to "real lengths", because of the lack of knowledge of physical objects to compare them to. Also, because after a while, these units become just "so many more zeros". So I would suggest the following:

Space Marginal Unit (SMU): 1,000,000 meters, or roughly the distance of from one end of France to the other. The minimum distance two spacecraft would have to be from each other before they would have to coordinate trajectories or go into docking manuvers. (Give me a bit of suspension of disbelief here folks.)

Length of Earth Orbit (LEO): 1,000,000,000,000 meters, the distance the Earth travels in one year. (The distance is actually about 6% less than that, but the LEO is something that can be visualized.)

Kaid: 1,000,000,000,000,000,000 meters. That's a bit more than the distance from here to the star Alkaid.

The above readily lend themselves to everyday conversation -- if we ever get to a point where we talk about such things everyday!

> **Comments:**
> **username_26**: I don't see how this answers the question. Also, LEO is the common abbreviation for *Low Earth Orbit*, which is something very unlike the Earth's orbit around the Sun.  
> **username_13**: "It's tough to relate something like a terameter to "real lengths"" Really? For me a parsec is equally tough to relate it to a length that I can feel. My simple view is that some stars and galaxies are just really, really far away. And that 1 terameter is clearly defined and therefore must have a meaning.  

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### Answer by username_36 on 2019-08-30. Score: 2

Because distance is **lumpy**. But bytes, booms, and buzzes vary **smoothly**.

The examples from the question, where metric prefixes became conventional -- terabytes, megatons, gigahertz -- are domains where human experience proceeded continuously across orders of magnitude.

* There were no hard, persistent thresholds in the [growth of hard drives, ICs, or cables](https://en.wikipedia.org/wiki/Moore%27s_law). Except for a little stickiness at the powers of 2, that progress was continuous.
* Explosions grew gradually over history. There were rare leaps such as nuclear weapons but there are no magic numbers. If every fusion bomb had the same yield then maybe that would have become a scientific unit, but [they varied all over the place](https://en.wikipedia.org/wiki/List_of_nuclear_weapons_tests#Most_powerful_tests).
* There are few magic frequencies long familiar to humans. Electromagnetic waves have a vivid island at [visible light](https://en.wikipedia.org/wiki/Visible_spectrum). But even that is smeared across an octave (400-800 TeraHertz) and there are wide oceans of unremarkable uniformity to either side.

Human acquaintance with **distance** on the other hand proceeded in fits and starts. "[We were bounded only by the earth, and the ocean, and the sky,](https://vimeo.com/2822787)" said [Sagan](https://youtu.be/YH3c1QZzRK4). Those hard boundaries on human travel persisted for millenia. The stride of an adult is an ancient, narrow, *familiar* island on the spectrum of distances.

# The distance to the sun was always familiar, and apparently large, long before anyone could measure it.

So terms for these distances persist, a meter, an Astronomical Unit. Perhaps it's arbitrary that everything longer isn't measured with metric prefixes in AU. But a "lightyear" (63 kilo-AU) does anchor that freakish distance onto two tangibles that could hardly be more familiar, even if their combination is not.

**Time** is another lumpy domain for humans, with deep ruts at the extent of a day, a year, a breath. So "gigasecond" (about 32 years) would be nonsense.

For distance and time, no single unit will do.


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### Answer by username_37 on 2018-06-14. Score: 2

The simple answer is: the larger units such as AU or light years are easier for the human brain to remember. And we should avoid putting units with to many zeros trailing after the first few digits, for example:1,000,000,000,000,000,000,000,000,000,000 meters. We could use AU, or for even higher distances, light years. If it was shorter, we cold still use meters but with an exponent.

> **Comments:**
> **username_0**: 1 AU is about 0.15 Tm, if you use the right prefix you don't have excessive zeroes. The size of a water molecule is 0.275 nm, we don't say 0.000000000275 meters.  

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