DEFINITION OF 1 METRE..... experts a tough one 4 u all?

Question

the metre is defined as the distance travelled by light in 1 / 299,792,458 th of a second.

why did'nt people choose some easier no. such as 1/300000000 th of a second or why not even 1 second ?

Answer 1

Before the speed of light was defined and before we used the speed of light to determine the length of the meter, there was a physical bar used as the standard for all meters (as there is for the kilogram today). When all the measurements were done, the speed of light came out to be pretty close to 299,792,458 m/s (different measurements would yield slightly different values due to associated experimental error).
It was decidedly easier to simply define the speed of light to be exactly 299,792,458 m/s and be done with it, and in turn, define the length of the meter as well.

The speed of light is a good thing to use in determining the length of the meter since no matter the frame of reference or wavelength of light, the speed of light is always thee same...an universal constant.

We can call the length of unit whatever we want, even the distance which light travels in a full second (rather than 1 / 299,792,458 th of a second), but then it doesn't make it a very practical unit to use on a daily basis. If we did that, then all of a sudden, people would use a multiple of the newly defined "nano" meters in place of what used to be meters.

Answer 2

Lots of good answers here. A bit of history: The French Academy of Sciences proposed that the meter be 1/10,000,000 of the meridian, passing through Paris, measured from the equator to the pole. Lots of surveying was done; the story has been recounted in a good book whose title I don't recall. Once the surveyers had finished, the prototype meter bar was prepared to be as close as possible to that length, and it has been kept ever since in a vault in Sevres, France. Replica meter bars have been made and can be found in a number of places including the Bureau of Standards (it isn't called that anymore -- they changed the name) in Washington DC.

As more precise measurements became necessary, it became useful to develop optical standards. The first such was to define the length in terms of a particular wavelength of light: 1,650,763.73 times the wavelenght of a specified orange red line in the emission spectrum of Krypton-86. Later it was decided that to base it on the speed of light made more sense than to use a wavelength.

Now, back to the French surveyors -- how good a job did they do? The currently accepted values for the size of the earth are: 6,378,137.000 meters, center to Equator; 6,356,752.314 meters, center to pole. Crank these into the formula for the length of an ellipse (which is approximate -- an exact closed form formula does not exist) and you get the meridian as 10,002,949 meters. Not too bad.

Answer 3

this would be because the metre existed before the speed of light existed, and the speed of light was then measured in metres. The definition of the meter is just 1 divided by the speed of light.
The metre was also designed in units that were divisable by 10...hence 1 cm = 10 * 10^-1, a km = 10 * 10^2
However, according to wikipedia, historically, the metre was intended to be, and is very nearly, the ten-millionth part of the distance from the equator to the north pole.

Answer 4

It was originally set up so that anyone could agree on the length of a meter, even if they couldn't come into direct contact. I believe the meter was originally defined as some fraction of the earth's circumference at the equator, but since then it has been refined slightly.

For a long time, it was the length of a platinum-alloy rod kept in a vault in France and under constant observation by laser-based measuring devices to detect any change in length (there hasn't ever been an observable change). Recently, though, they have started going back to basing the metric system on natural phenomena, and the meter has been redefined as a particular fraction of a light-second.

Answer 5

The reason for this is historical. The meter *used* to be defined as the length of a particular piece of metal somewhere. (A platinum/iridium bar somewhere in France I think.) The modern definition was chosen to be compatible with the old definition!

The first suggestion is reasonable, since it's pretty close to the standard meter. However, light travels a *long* way in a second! This wouldn't be a good unit for everyday measurements.

Answer 6

Well, in the 18th century the French Academy of Sciences (I think) declared a metre to be 1/10 000 000 of the earth's meridian (measured how and in what I do not know). This turned out to be inaccurate, and after a series of changes in how the metre is defined, we have reached our present definition.