265.296 Metres

To kick off my series about length measures and the Google Maps DMT, I decided to start with one of the more common (and, arguably, useful) units on offer: the metre (m).

The metre (or meter to our American cousins) is one of the seven basic units of the SI system of measurements favoured by the world scientific community, which is the modern form of the metric system.

There have been several definitions of the metre since it was first proposed by English philosopher John Wilkins in 1668.  Apparently Wilkins proposed setting 1m to be the length of a pendulum with a half-period (IIRC that’s the time taken to swing from one extreme to the other – a full period being the time to complete a swing there and back again) of 1 second.  During the eighteenth century this original definition vied with another that defined the metre as one ten-millionth of the length of the Earth’s meridian along a quadrant (that is the distance from the equator to the North Pole) at sea level.

When the French Academy of Sciences defined their metric system in 1791 (around the time of the French Revolution), they opted for the latter definition, since the length of the pendulum required to give a 1s swing is affected by the slight variations in strength of the earth’s gravitational field at different location while the length of the meridian is constant (assuming, of course, you can measure it with sufficient accuracy in the first place).  As far as I can tell, this was the first official definition of the meter.

For nearly a century, from 1875, the length of the metre was defined to be the length of a specific metal bar measured under specified conditions (the gory details, along with a lot more information about historical definitions of the metre, can be found in the Wikipedia article linked above).  This was replaced in 1960 by a definition based on the wavelength of a particular line in the emission spectrum of Krypton-86 in a vacuum (presumably a constant for all Kr-86 atoms and therefore a, theoretically, more easily transferable measurement than the length of a single metal bar kept in Paris).

In 1983 the current definition of the meter was agreed, namely the length of the path travelled by light in vacuum during a time interval of 1  ⁄   299,792,458 of a second.  This is, of course, dependent on the definition of a second (which I won’t go into now, as this is supposed to be an article about length measurements) and the constancy of the speed of light in a vacuum (one of the cornerstones of Einstein’s theories of Relativity).

As an SI unit, the metre comes supplied with a whole bucketload of standard prefixes to denote decimal multiples of the basic unit.  This means that you can write distances from the very small (e.g. the diameter of a helium atom is about 0.0000000001m or 0.1nm (nanometres)) to the very large (e.g. the diameter of the sun is about 1,400,000,000m or 1.4GM (gigametres)) without excessive leading or trailing zeroes.  For everyday purposes, kilometres (1,000m) and millimetres (0.001m) are especially useful.

The span of the Menai Suspension Bridge (shore to shore, as measured using the Google DMT) is 265.296m.  My regular commute to work (using approximate start and end points, measured using the Google Maps navigational tools and my usual route) is 4.2km.

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