It might not seem something as constant as length would have a history, but it does.

In fact, it goes back to the times of ancient civilizations.

To begin with, you can’t really specify the length of something without comparing it to the length of something else. Consider money: without it, something may be valuable, but it can’t have a price. In today’s world, we use centimetres and inches as our standards, but that hasn’t always been the case.

If we go right back to the beginning, when we lived as nomads and hunter-gatherers, there really was no need for measurement. Phrases such as “two times my height” and “just slightly larger than a cow” sufficed. But, when we began to settle down and plant fields, descriptions like “my land is seven times my height long and ten cows wide” just weren’t enough any more.

Many ancient measuring systems use parts of the human body. It seemed logical enough at the time; people were not hard to find and there was nothing more to it than people. The problems arose when people began to realise that different people had body parts of different sizes.

So they chose one human to use as a standard: the King. (Jokes could be made here about how the Ruler became the first ruler, but I shall resist the temptation to do so.) Needless to say, the King could not drop all his duties and become a ruler instead. So the problem was solved by cutting various objects down to size to use as rough measuring sticks. Variations include wooden rods, Ancient Egypt and Mesopotamia, and ivory, Indus Valley, among other things.

Soon, a system was devised so larger units could be split into smaller lengths. Take Egypt, for example. The main length they had was the meh nedjes, or common cubit, which was the length from the elbow to the tip of the middle finger, and added up to about a foot and a half. A meh nedjes could be divided into six shesep, the width of the palm minus the thumb and a shesep consisted of four djebin, each of which was the width of a finger.

Other kingdoms were also setting up their own forms of measurement. Usually, a set of lengths was chosen based on needs and convenience, and a master copy made. These master rulers were stored in public areas, so people could use them to create other rulers and tools.

However, as people from across the globe began trading with one other, things quickly got confusing while converting between zhang and meh-ta and every other region-specific unit. So, in 1960, several countries got together and released the Systeme Internationl d’Unites, or International System of Units — also known as SI for short. Together, the countries set up standard units not only for length, but for weight and time as well.

They chose the metric system, originally from France, because of its ease of conversion. Ten millimetres make a centimetre, ten centimetres make a decimetre, and so on. Originally, they also had a platinum master copy of the metre in Paris. Platinum does not change shape very easily — but since it would be in use for so long, there was some fear that, over time, slight changes could happen that would throw the entire system off. Now, length is measured by something that will definitely remain constant: the speed of light. One metre is now defined as the distance light travels in 1/299792458 of a second.

The story of length has come a long way. But how long? That’s something which, unfortunately, cannot be measured with a ruler.

An earlier version of this article was published in Sirius #214 6 — 19 Mar 2016 “The Length of Things”, under the same title.

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