Question

Explain this common observation clearly : If you look out of the window of a fast moving train, the nearby trees, houses etc. seem to move rapidly in a direction opposite to the train’s motion, but the distant objects (hill tops, the Moon, the stars etc.) seem to be stationary. (In fact, since you are aware that you are moving, these distant objects seem to move with you).

Answer 1

Explanation of the observation:

When you are sitting in a fast moving train, your body and eyes are moving along with the train. The apparent motion of outside objects is a result of relative motion and change in viewing angle.

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Nearby objects:

Nearby objects such as trees, houses, and electric poles are at a small distance from you.

As the train moves forward, the direction of the line of sight to these nearby objects changes very rapidly.

Because of this rapid change in viewing angle, nearby objects appear to move quickly in the direction opposite to the train’s motion.

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Distant objects:

Distant objects like hilltops, the Moon, and stars are at extremely large distances from the observer.

For these objects, the change in the direction of the line of sight due to the train’s motion is extremely small.

As a result, their angular displacement is negligible, and they appear almost stationary.

Since you know that you yourself are moving, these distant objects may even appear to move along with you.

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Key idea (Parallax effect):

This phenomenon is an example of parallax.

Objects closer to the observer show larger parallax and hence larger apparent motion, while very distant objects show negligible parallax and appear stationary.

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Conclusion:

Thus, nearby objects appear to move rapidly in the opposite direction due to large angular change, whereas distant objects appear stationary due to negligible angular change caused by the observer’s motion.