Where did the boat leave?

Imagine that an oar boat is sailing along the lake, and let, for example, arrow a represent the direction and speed of its movement. A sailboat is passing by; Arrow b shows its direction and speed. If you are asked where the boat sailed, you, of course, immediately point at right angles to the rowing boat point M on the shore. But if you ask the passengers of a rowboat with the same question, they would point to a completely different point. Why?

This is due to the fact that passengers see a boat moving not at right angles to the path of their boat. They do not feel their own movement: they think that they themselves are standing still, but everything is moving at their own speed, but in the opposite direction.

Therefore, for them the sailing boat moves not only in the direction of the arrow b, but also in the direction of some dotted line a, back to the motion of the rowing boat. Both movements of the sailing boat - real and apparent - are built according to the parallelogram rule. As a result, it seems to the passengers of the boat that the sailing boat is moving along the diagonal of the parallelogram, built on b and a. That is why it seems to passengers that the sailing boat sailed from the shore not at the point M, but at some point N, far ahead in the motion of the rowing boat.

Moving along with the Earth in its orbit and meeting the rays of stars, we judge the position of the sources of these rays as incorrectly as the passengers of the rowing boat erroneously determine the place of departure of the sailing boat. Therefore, the stars seem to us a little shifted forward along the path of the Earth's motion. Of course, the speed of the Earth's motion is negligible compared to the speed of light (10000 times less); So the apparent displacement of the stars is insignificant. But it can be detected using astronomical instruments. This phenomenon is called the aberration of light.