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Consider a receiver on Earth measuring radio waves from an incoming slow moving UFO.

Here is my attempt to find the period of the signal as measured by the earth observer. T' is the period of the earth receiver, T is the actual period of the signal.
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In the first reference frame, with the UFO stationary, the receiver appears to intercept each signal prematurely, and so measures a reduced period of the incoming signals.

In the second case, UFO moves and the earth is stationary, the UFO moves after it has released its first signal by an amount VT, before it releases its second signal. The distance between the 2 signals is CT-VT, which is the apparent wavelength according to the earth observer. The two dont give the same period measured. Where have I gone wrong?

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