Why does the moon follow your car?

The moon is about 384,000 km from Earth on average. Even if you drive about 140 km in a straight line, roughly the distance from Seoul to Daejeon, the direction you look at the moon changes by only about 0.02 degrees. The moon is about 0.5 degrees wide in the sky, so that's a shift of about 1/25 of its width.

That's why, even after driving for a long time, the moon is still in the same corner of the window. The moon didn't drive along with you; the distance you traveled was just far too short compared with the distance to the moon.

The closer an object is, the more its direction changes as you move. Roadside trees flip from front to back in the blink of an eye, distant mountains slide by slowly, and the moon barely moves at all. This difference in apparent motion depending on distance is called motion parallax.

When the scenery around you flows backward, your brain concludes that you are moving forward. But since the moon alone doesn't slip back and stays in the same spot, your brain takes it that the moon is moving along with you. That's where the feeling of being followed comes from.

In 1924, Swiss psychologist Jean Piaget asked about 60 children aged 4 to 12 living in Geneva nothing more than "What does the sun do when you go for a walk?" Many of them answered without hesitation that the sun and moon follow them, and stop when they stop.

Piaget wrote that this belief lasts until about age 8 on average. Around ages 8 to 10, children mix the two ideas, explaining for example that the sun stays still but its light follows them, and only at about 11 to 12 do they understand that it merely looks that way because it is far away.

It's not that the moon never moves across the sky. Because the Earth rotates, the moon drifts west by its own width about every 2 minutes. If the moon's position has changed after driving for more than an hour, that's thanks to the Earth turning, not the car.

You can check this at home too. Hold out your arm with a finger raised and close one eye at a time. Your finger jumps left and right from a difference of just over 6 cm between your eyes, but the moon outside the window doesn't budge.