Parallactic effect while driving


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Wherever I go, the Moon is following me!!

It is an experience surprisingly shared by many: Have you ever felt like the Moon was following you as you drive along a straight road at night? Are we really all that special or is there a simple explanation to this very common perception?

If you carefully look at the following animation, you will notice that, as our character drives along the road, things in the picture do not apparently move at the same speed. Right by the car, the roadside seems to fly by, while the trees, a little further away do seem to move a little slower. The far away mountains also drift, albeit very slowly.

Consider this, the distance that separates the road from the mountains is only of a few dozens of kilometers and they already barely seem to move as one drives by. With this in mind, are you really that surprised that the Moon (400,000 km away) and the stars (light years away) do seem immobile?

Of course, if you wait long enough, because of the Earth’s spin, the postion of the Moon and the stars will begin to drift. But let’s assume that for a few minutes, they don’t. As our character drives along the road, and covers some distance, by looking at how much the mountains have drifted in relation with the “fixed” sky (measuring an angle), he should be able to tell how far the mountain range is from the road.

In astronomy, this principle is used to figure out the distance to the most nearby stars.