KnowHOW team explains: The huge distance between two heavenly bodies leads to a number of phenomena that surprise us. The moon’s apparent walk with us is one of them.

To understand how the distance between two stellar objects affects their apparent motion, consider the movement of a star relative to us on earth at a distance of 1018 (one followed by 18 zeros) metres. Even we move 10 km on earth, the angle of sight of the star changes by an extremely small amount. The change is so small that it cannot be noticed but for accurate measurements. The same thing applies to the moon which is 4x108 metres from the earth. The moon can be seen at the same angle, even when we move 10 kilometres.

On the other hand, a cloud is just a few kilometres away from us and appears to have an angular deviation even if we walk through just a kilometre. The distance being small, the angular deviation is appreciable in this case. Thus clouds appear to advance or recede with respect to us when we walk.

When we see the moon, in all likelihood we see it with stars in the background and clouds passing in the foreground. If we mentally fix a reference to the cloud, we feel we are moving with respect to it. But with the moon, this is not the case ? the reason we feel the moon is moving with us.

The question was sent by Anantha Rao from Hyderabad