# I cannot understand $\tau_{see}$ means in the question

The question is

Time dilation implies that if a clock moves relative to a frame S, careful measurements made by observers in S [as in Fig. 1.22(a), for example] will find that the clock runs slow. This is not at all the same thing as saying that a single observer in S will see the clock running slow; and the latter statement is, in fact, not always true. To understand this, remember that what we see is determined by the light as it arrives at our eyes. Consider the observer Q in Fig. 1.22(b) and suppose that as the clock moves from A to B, it registers the passage of a time $$\tau_0$$. As measured in S, the time between these two events (“clock at A” and “clock at B”) is of course $$\tau$$ = $$\gamma$$ $$\tau_0$$.However, B is closer to Q than A is; thus light from the clock when at B will reach Q in a shorter time than will light from the clock when at A. Therefore, the time $$\tau_{see}$$ between Q’s seeing the clock at A and seeing it at B is less than $$\tau$$