I respectfully disagree (without attempting to say you're wrong!) about the definition of coincidence and the requirement of being non causally related. If I'm riding on a bus and the light poles going past line up with my music, that's a coincidence even though they are cause soley by the bus motion BPM matching an essentially random choice of song BPM.
What I'm describing is an artifact in your data that is caused by the motion of the earth.
To give a more concrete example, suppose you measuring the brightness of a trans-neptunian object, and observe that the brightness changes slightly with a period of about 1 year. You might think it has a non-uniform albedo, and a rotational period of one year, when in reality, it is just brighter when the earth is closer to it.
