What is 'c' ?

This question appeared in the sci.physics.relativity newsgroup:

"My question is - WHY? What makes the speed of light to be this and not
 let's say - 1 million km/sec? What physical property in the universe makes
 this limit to be as it is?"

Suppose we are given the shorter sides of a right-angled triangle as 6 km and 5 miles. First we change the value in miles to km using the conversion factor 1.6. The longer side is then 10 km. We could include the scale factor by modifying Pythagoras equation as:

where x is in km, y is in miles and a is the conversion factor, 1.6. The purpose of the constant 'c' is the same - to convert a temporal measurement given in seconds into units of metres, just multiply by 299,792,458.

In fact, this value forms the definition of the metre:

in the same way that we define the inch by:

The reason for c having the specific value it does comes purely from the historical origin of the units in which it is expressed.

Since space and time are the same thing viewed from different angles, they must be measured using the same units. The scale factor 'c' is only needed to correct for our use of mixed units.