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**very far**from the origin of coordinates let there be a sourcce of electromagnetic waves..." . Taking into account his advise "never stop thinking" we could ask if the Doppler shift formula he derives holds in the case of scenarios which do not fulfil the "very large source-receiver" assumption.

Consider a scenario proposed by A.P.French,

*Special Relativity" (Nelson 1968) pp 141-143*that involves a satellite (plane) that moves at a constant altitude emitting light signals at constant time intervals which are received by a stationary observer located on Earth. In order to recover Einstein's formula, French makes the assumption: ' The satellite travels a

**very small distance during one cycle of its transmitter signal**which is equivalent with Einstein's assmption or with the assumption that

**the period at which the successive liht signals are emitted is very small (the frequency is very high)**. Take into account that we can emit successive light signals at lowmechanical frequencies.

My question is: Before using a ready derived formula following a given scenario, should we investigate if the assumtions made deriving it fit in the case of the scenario we follow.

My oppinion is that we should, deriving shift formulas in accordance with the scenario we follow showing how flexible special relativity theory is. What is your oppinion?

**Sine ira et studio**