Explaining the aberration of star light

Imagine that we are standing on a platform inside in a flowing river. Imagine that the river is flowing towards the south and we are facing towards the north. If the platform is stationary, we would obviously feel the water coming from the north. But if the platform moves towards the east, we would feel the water hitting our body from the north-east direction despite the fact that the water is flowing in the same direction as before. And similarly if the platform moves towards the west, we would feel the force of water coming from the north-west direction. If we close our eyes and are unaware of the motion of the platform, we would think that the water flow is actually coming from the north-east direction when the platform moves east ward and conversely when the platform moves westward, we would think that the water flow is coming from the north-west direction.



If we are asked to fill a pot with water, we would obviously face the mouth of the pot north ward when we the platform is stationary, north-east ward when the platform moves towards the east and north-west ward when the platform moves towards the west.



That explains why we have to tilt our telescope depending upon the direction of the Earth’s motion.



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