It is emitting a signal of frequency ν0 in S. (I am here using the frame “in S” as earlier in the chapter to mean “referred to a reference frame in which S is at rest.) The signal arrives at...It is emitting a signal of frequency ν0 in S. (I am here using the frame “in S” as earlier in the chapter to mean “referred to a reference frame in which S is at rest.) The signal arrives at the observer O at a slightly greater frequency as a result of the decreasing distance of S from O, and at a slightly lesser frequency as a result of the time dilation, the two effects opposing each other.
