meterstick การใช้

But you would measure the speed of light relative to the average length of your metersticks.
In his original paper, he constructed coordinate systems explicitly ( from clocks and metersticks ) and didn't call them " observers ".
:: : Don't confuse an inertial frame ( which you can think of as a huge network of clocks and metersticks, filling a large volume of space ) with a person at a particular location recording the light that enters his / her eyes.
Einstein carefully distinguished the two in his original 1905 paper on special relativity, using " observer " only for the person, but almost every later writer uses " observer " for the network of clocks and metersticks, even though that doesn't agree with the ordinary meaning of the word.
To say that " the speed of light is constant " is to say that \ sqrt { ( \ Delta x ) ^ 2 + ( \ Delta y ) ^ 2 + ( \ Delta z ) ^ 2 } / \ Delta t is a constant ( namely c ) for straight-line motion, where x, y, z, t are coordinates defined by some inertial reference frame ( a network of metersticks and Einstein-synchronized clocks ).