. However, for reasons which will become apparent in the theory of relativity, it is advantageous, though by no means imperative, to select the same unit of length in our graph in order to represent both one second in time and 186,000 miles in distance.
Fig. IX
Suppose, for instance, we agree to represent these magnitudes by a length of one inch on our graph; then a point such as
, one inch from
and one inch from