Why don't astronomers use meters to measure astronomical distances?

Question

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?

Answer 1

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. 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. 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.

Answer 2

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.

Answer 3

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.

Answer 4

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 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 gerrit: 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-.

Answer 5

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 6

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.

Answer 7

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 $1 km$.

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.

Answer 8

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.

Answer 9

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.

Answer 10

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!

Answer 11

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. 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.

• There are few magic frequencies long familiar to humans. Electromagnetic waves have a vivid island at visible light. 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," said Sagan. 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.

Answer 12

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.