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Consistent units are one of the most important inventions to the modern age.

Before standardization, one used natural units such as 'feet', but these all varied from person to person.

My world abides to all natural laws of Earth and has developed very similarly.

Could anything exist naturally in my world that could serve as a consistent unit of distance measurement?

Could there be a plant, mineral or other natural occurrence that grows to a certain, consistent size?

Ideally, a unit should be roughly equivalent to a metre.

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    $\begingroup$ I believe that the only truly universal measure of distance is the Planck length. It is, however, a teeeny bit shorter than a metre. en.wikipedia.org/wiki/Planck_length $\endgroup$
    – Klaus Æ. Mogensen
    Oct 15, 2019 at 13:01
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    $\begingroup$ The metre has an accurate (as accurate as possible within the limits of our instruments) and reproducible definition in terms of natural constants. (And the foot is defined in terms of the metre, so I'm not at all certain which of the two is more "natural".) ("Before standardization" is more than two and a half thousand years ago. The Hellenistic and Roman civilizations had standardized units of measurement exemplified by metrological etalons, just as we do. Their accuracy was not all that great by modern expectations, but it was in line with the accuracy of their instruments in general.) $\endgroup$
    – AlexP
    Oct 15, 2019 at 14:06
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    $\begingroup$ The speed of sound at STP, because I thought this was going to be about music. $\endgroup$
    – Mazura
    Oct 16, 2019 at 2:54
  • $\begingroup$ Is there a reason why the standard we use, the distance of the speed of light for a specific fraction of a second, isn't suitable for this world? $\endgroup$
    – billpg
    Oct 16, 2019 at 10:30
  • $\begingroup$ @billpg The inhabitants may not have the technology to determine this value. Ideally, it would be a unit accessible to prehistoric cultures. $\endgroup$
    – A Lambent Eye
    Oct 16, 2019 at 10:40

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The use of carats as unit of weight comes from the relatively narrow distribution of weight of the seed of carob

The unit "carat", used for weighing precious metal and stones, also comes from κεράτιον, as alluding to an ancient practice of weighing gold and gemstones against the seeds of the carob tree by people in the Middle East. The system was eventually standardized, and one carat was fixed at 0.2 gram.

Since also the distribution of dimension of the seeds is pretty narrow, you can use the seeds as unit of length. The length of about 100 seeds should be somewhere around the length of 1 meter. (image source)

carob seeds

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  • $\begingroup$ Actually, the seeds did have similar variance as others (see the sources from Carat. Though of course in a new world you might have a seed which has a more consistent mass. $\endgroup$
    – Paŭlo Ebermann
    Oct 15, 2019 at 23:01
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Capillary action

When hollow tube is submerged to a fluid, the surface level inside tube is different from the outside, as if defying gravity. This difference is universal and can be used to set a standard length. For example with glass tube in water, in a 3.85 mm radius tube, the water would rise 3.85 mm. No other tube diameter can produce the ratio of 1 between diameter and height of meniscus.

From this meter standard can be derived when you place 260 inner diameters of such tube (presumably, casts of it) next to each other.

Casting metal alloy meter lengths, or marking bones centrally and distributing them seems more practical.

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  • $\begingroup$ Different material for glass/water/air will chang ethat universal constant, won't they? $\endgroup$
    – Hagen von Eitzen
    Oct 16, 2019 at 8:50
  • $\begingroup$ If they can't consistently produce one type of glass and clean water, they have much bigger problems, I guess... $\endgroup$
    – Juraj
    Oct 16, 2019 at 10:05
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    $\begingroup$ How would one produce a tube of a certain diameter without being able to measure the diameter? $\endgroup$
    – A Lambent Eye
    Oct 16, 2019 at 10:41
  • $\begingroup$ @ALambentEye The point is that you try tubes of various sizes, and when the meniscus height of water in your tube is equal to the diameter of the tube (which you can verify with a piece of string or something), you know that the tube's inner diameter is 3.85mm. Now that you have something you know to be 3.85mm, you can measure out 260 of them to get something that is 1.001m long. $\endgroup$
    – anaximander
    Oct 16, 2019 at 14:46
  • $\begingroup$ Oh I see! That is very clever. $\endgroup$
    – A Lambent Eye
    Oct 16, 2019 at 16:34
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If your culture is scientifically advanced, a truly uiversal length unit is the Planck length. which is ca. 1.616×10-35 m. Using 1035 times this length gives you a suitable length of 1.616 m.

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The short answer is "anything." All you need is an evolutionary pressure to be X meters tall but not X+1 meters tall, and natural selection will take over. Balances in natural things have proven remarkably precise. Averages work even better. If you can measure the length of 100 things end-to-end, you can use the central limit theorem to get a more accurate measure. This is why Carab seeds are so effective for measuring mass, over say large rocks. They're nice and small.

One more precise answer would be to use the circumference of the planet. The meter was originally defined to be one ten-millionth of the distance from the pole to the equator. Such a measure would technically meet your needs, as it is measuring something natural.

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  • $\begingroup$ Could you give an example of such an 'evolutionary pressure'. I can't find anything tangible that would result in a specific length $\endgroup$
    – A Lambent Eye
    Oct 17, 2019 at 11:06
  • $\begingroup$ @ALambentEye I could see one forming from a need for a resonator at a certain frequency. Or one which is limited by muscle strength (which is affected by a square-cube law that could set a value). That being said, I answered this before seeing that you wanted an accuracy of 1 part per 100 (1cm). Not even the carob seed, famous for its regularity, could do that reasonably well. I'd recommend a metrology style calibration process, myself. Register your meter stick against that of the local lord. $\endgroup$
    – Cort Ammon
    Oct 17, 2019 at 15:22
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A large femur (thigh-bone) of an individual creature.

Bones are relatively weather resistant and can last for a long time, so a particular femur off of a historical hunt may be passed on, similar to the metre bar.

There are a few disadvantages to this solution:

  1. It is unique and may become damaged, making it worthless.
  2. It is unique and is therefore not readily available, unlike a 'foot'.
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