This is being asked from a lay-person point of view. However, please do not hesitate to give more detailed answers that require prior knowledge - if these answer the question more accurately.
My confusion currently comes from the fact that everything in the universe is moving in some way or another - so any reference point we use would also be moving/not completely reliable.
As such, how can we accurately determine that we have completed an exact 360 degree rotation around the sun (and as such, measure exactly how long it took)?
To be clear, this is asking about how we can determine that we have completed a single 360 orbit of our sun, and not the timing aspect of how we know how long a second is. If this situation is different for measuring the orbits of other bodies around their star - it would be great to expand on those differences as well.