# Why don't they change the size of a meter so that the speed of light is exactly 300,000 km/sec?

The speed of light is 299,792 km/sec. That is less than 1/10% away from 300,000. Nobody would notice the change, but it would be nice to have a simple number for the speed of light.

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I could ask the same about why don’t they change a foot to ten inches and a yard to ten feet, so all I have to do is move a decimal to figure out measurements.

I’d like them to make ten ounces in a cup and a pound while they are at it.

It is what it is.

Are you sure that 299792 km/sec is the EXACT number?? Think of all the other things that would have to be modified to even EVERYTHING out. Would it be worth the trouble??

What @kritiper wrote. It would not be a good idea to change the size of a meter, meaning all the endless things we measure in meters and so on, would be inaccurate, and if you wanted more than 1/10% precision on any measurement (which you would, at least for some engineering tasks and other precise matters, as well as practically every scientific book and reference written in the last 100 years), then you would need to make sure whether you were referring to a source before or after the change.

Spacecraft would probably end up crashing because occasionally someone would get some number wrong. Etc.

The speed of light is difficult to measure with accuracy and is one of the least-used figures there is, and the people who use it are also since 1960 well-used to it being 299,792,462 ± 18 meters per second, so they wouldn’t thank you either.

For the history of what the meter was defined as over the decades, and why, see for example this rather good NIST article.

If it makes you feel better most engineering and physics textbooks tell you to use 3E8m/s for the speed of light in general calculations.

I did a Web search and found that the official definition of a meter uses the speed of light to be exactly 299,792,458 m/sec. They could have used 300,000,000 m/sec, but apparently found that to be too far from the previous definition of a meter.

Wikipedia Link

There is no reason apart from convenience. Just think how often metres centimetres and millimetres are used in daily life compared with the speed of light. If the length of a metre were changed it would cause endless confusion. You would always be asking if measurements were made in old metres or new metres and having to convert from one to the other.

How do you change the size of a meter, when it is used as a measurement in a zillion places, locations, scientific work, manufacturing, etc. You can’t just change a uinit of measure.

A meter is an artifically ceated measurement and so its use in everyday life is worthless. A meter is too big for the average measurement around the house or on a construction site, a centimeter is too small for quotidian uses.

Really, other than an astrophycisicst, who gives a F- what the relation between the speed of light and a meter is? When was the last time the accuracy of the speed of light came up in your life?

the exact value is defined as 299792458 metres per second

It’s a Space thing to be that exact.

Why is it the way it is

The speed of light is constant. There are no relativistic effects. If you move toward a light source or away from it, you will get the same value for the speed of light. This made it very convenient to determine the length of a meter. Of course you also need to calculate time, which is a little messier, but had to be done, since we need a standard for time.

There must have been a standard for a meter before using the speed of light. What they must have done was to round the speed of light to the nearest whole number using the old meter standard and used that to determine the new standard for a meter, so that the speed of light in m/sec is an exact whole number.. Since the speed of light in meters/second is a 9 digit number, absolutely nobody would have noticed the difference.

Actually light speed is only constant in a vacuum. It slows down in other mediums.

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