Why is kilometer the biggest metric unit used?
For example, Alpha Centauri is 41.32 trillion kilometers away. Why isn't this expressed in a larger unit like megameters, gigameters, or terameters?
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answers:
Gabe say: At that scale, may as well switch to Planetary Radii (distance from a Planet's Surface to center), Astronomical Units (Earth-Sun distance) and Light Years
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Raymond say: In scientific papers, the bigger units are used. Specially if you have to use them in calculations (I used such units when I wrote about gravitational waves caused by stars orbiting each other).

However, with time, we have seen more the use of the metre, with scientific representation of numbers such as 4.5*10^15 m. These numbers can be directly thrown into equations.

We try to avoid words like billion and trillion which are ambiguous (there are two different scales for naming numbers -- the one used by the majority of people is NOT the one used in the USA.

The only exception that we see all the time is 13.8 billion years (using the US billion, which is a milliard in the rest of the world) for the apparent age of the Observable Universe.
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Jim Moor say: It's similar to saying a city is 15km away, not 15,000m.
In metric, for long distances, you typically use kilometers.

That said, there are 2 basic "system" used in scientific study:
KMS which stands for Kilogram-Meter-Second units
and CGS which are centimeter-gram-second units
So a scientist working problems in the norrmal world would always use Meters, not kilometers.

But stating a long distance, we'd us km to knock off 3 zeros.

We COULD use gigameters! But let's face the facts, a lot of people don't learn the metric system, just like the US doesn't learn the US Customary system!
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Krishnamurthy say: The kilometer was originally defined to be one ten thousandth of the distance between the North Pole and the Equator through Paris.
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SBR32277 say: I suspect the main reason is because of formulas that use an agreed upon standard units SU. It's not that other units are never used but if you are going to be using those numbers in other formulas, it's common practice to simply state those numbers in standard units so that you don't have to convert them into standard units.
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David say: Very large numbers involving great distances use 'scientific notation' as for examples 1,000,000,000,000,000 kilometers in scientific notation is 1.0*10^15 kilometers
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busterwasmycat say: They do get used. We often use km though because that is about the maximum unit that provides us with a visceral understanding of its size. People understand how far a kilometer is in some relative mental image. When you slap a bunch of zeroes on a number the only thing the average person gets out of it is "really freaking big".

This is not unlike how large volumes can sometimes be expressed as "x olympic swimming pools", or on the other end, "smaller than a human hair".
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billrussell42 say: It could be, but for some reason, those units are not used much.

Units in use are exponential notation, such as 41.32x10¹⁵ m, which are preferred to terms like "trillion" which is ambiguous.

There is also light years, and parsecs for astronomical use.
1 LY = 5.88 x 10^12 m
1 pc = 3.26 LY

ditto with Mg, Gg for mass.
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Vaman say: We have a bigger units. Light year= distance the light travels in a year. We have par sec. This is the second of arc made by the diameter of the Earth orbit around the Sun. We do not use mega,giga etc for the distances.
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say: Astronomical distances ares usually given in light years or AU's.

Km are often used in that context to give a sense of awe and grandeur.
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CarolOklaNola say: Try parsecs. A parsec is 3.26 light years.

A light year is unit of distance.

I STILL "think" in English/ imperial u its even after 40 years using BOTH. Most scientific papers use other sets if units unless its medical where centimeters grams and seconds are often used instead meters or kilometers, kilograms and seconds. A square mile compared to a hectare. And then there are arpents, which the French used.
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Who say: you wanna specify astronomical distances using kmtrs is up to you

I prefer light years
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khalil say: kilometer and mile were standardized for the distance between cities.
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Lôn say: Excellent question, I really don't know the answer .
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Markus Imhof say: That distance is usually expressed in lightyears or (kilo-, mega-, giga-) parsecs - if it is expressed in words at all. Scientifically, it would be expressed in exponential notation.

Those "xx trillion km" belong in the realm of journalistic expression, when journalists try to express something they don't understand themselves in a way they think their reders may be able to relate to - similar to measuring area in the size of football fields, or comparing (often over here) the area burned down in a large forest fire to the area of a local state.
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