Christopher Joseph - How to Measure Anything: The Science of Measurement

Although the famous bluestone slabs arrived at Stonehenge in the west of England from Wales in around 2500 B.C.E., the earliest similar construction on the site is believed to have been built some 600 years earlier.

Ptolemaic system

In his astronomical treatise Almagest, one of the most influential books of Antiquity, Ptolemy compiled the astronomical knowledge of the ancient Greek and Babylonian world. This geocentric model of the solar system remained the generally accepted model in the western and Arab worlds until it was superseded by the heliocentric solar system of Copernicus.

For centuries after the fall of Rome, systems of customary units remained largely unchanged; there was, after all, little new to measure. Still, the feudal monarchs of Europe were interested in measurement for the same reason that any government is: if you do not know what someone owns or produces, how can you tax them on it?

As the Renaissance reinvigorated progress in art and science, however, interest in measurement regained some of its former vigor. Traditional units, however, continued in usesometimes more different from one town to its neighbour than they were from one century to the next. Steadily, though, they became more accurate and reliable. Candles with hour markings were replaced by clockwork. While a clock in one town might tell a different time from that in a town 20 miles away, identical clocks could at least be relied upon to keep each hour to the same length.

Then came the French Revolution, and the great reform heralded by the introduction of the metric system. Though initially unpopular even in France, this system was the logical measuring accompaniment to the decimal Arabic numerals that had by then firmly supplanted Roman numbers throughout Europe. Decimal numbers made calculations of all kinds far more straightforward, as well as greatly simplifying much mathematical thinking. Trade and measurement, however, was another matter: 10 sheep were 10 sheep, but a pound was still 12 ounces. Conversions between different types of measurement were more complex stillif one pint of something weighed one pound, one gallon of it most definitely did not weigh one stone. The metric system supplanted all of this with a system of related units, distinctions of scale being made purely by the use of prefixes to indicate the magnitude of the number.

The standard unit forming the base of this system, from which all of the others were initially derived, was the meter. It was defined as one-ten-millionth of the distance from the Equator to the North Pole along a meridian passing through Paris (in a steady curve at a theoretical sea level, rather than along the less-than-smooth surface of Earth). An enormous survey was carried out at great expense to calculate this distance as accurately as possible, and a platinum-iridium barthe equivalent of Pharaohs master cubitcast as a permanent record. Carefully manufactured duplicates were distributed throughout Europe (and, ultimately, the world) to ensure that everyone was using the same meter.