Why does the moon sometimes seem to follow us?

It is night, you are traveling in a car while someone else is driving, and when you look out the window, you suddenly have the feeling that the moon is following you. Do you know why you experience this strange sensation?

This experience is frequent and children comment on it from time to time. That is why it is often approached in scientific dissemination forums. Today we will try to go one step further in understanding it.

How we perceive movement

Our displacement is appreciable with respect to nearby things (the environment of each moment) but not with respect to the moon, because of the enormous distance that separates us from it.

To this physical-optical explanation we can add some psychological factors, that is, the way our mind interprets the situation. If it is the environment that moves in our visual field (things –such as trees, houses or street lamps– change position and become smaller), why do we assume that it is the moon that moves and not the environment? For this the psychology of perception has two complementary explanations.

Everything flows when we move

When the whole environment moves in our visual field, following a common pattern, this is called “optical flow”. It is a basic concept of the “ecological optics” of the psychologist JJ Gibson, who was interested in the relationship between our perceptions and our actions (from something as simple as turning the head or running, to something as complex as piloting an airplane).

The optical flow of things, that is, their displacement in our visual field, is an indicator that we are the ones who are really moving. This helps us guide our movement.

Then he will think: “Sure, I know that I move because I feel my movement, I don’t need optical flow for that.” But then look at situations where you are stationary and the optical flow is sufficient to create the sensation that you are moving. For example, a YouTube video recorded with cameras that a cyclist is wearing on his helmet, does it not make him feel that he is moving like the protagonist?

Thanks to this ability we have to interpret the optical flow, the cinema can create the illusion of placing ourselves in the character’s point of view. In the film Top SecretFor example, there is a funny example of how to create an impression thanks to optical flow… in this case wrong!

Objects escaping the optical flow

But what does all this have to do with the moon? Well, objects that do not follow the global pattern of optical flow created by our own movement indicate that they are moving by themselves (Gibson called them “local disturbances”). Therefore, since the moon does not follow the same optical flow pattern as the rest of the scene, it seems to move and follow us.

To see it, think, for example, of a car following you when you are in a caravan. Would I watch him walk away and get smaller? No, he would still look the same size and distance all the time.

It would constitute a “local disturbance” in the optical flow and that would indicate, according to Gibson, that this car is moving at the same speed as ours. The same conclusion draws our perception regarding the moon.

Induced movement (or how to blame someone else for what happens)

The second explanation, which does not contradict the first but complements it, takes the context into account. Our perception is guided by “heuristics” or rules of interpretation, which are the “default options” that the brain assumes when it is difficult to interpret what is in our visual field.

If in our visual field there is a small object, framed by a larger one that serves as a frame or context, and a displacement changes their relative positions, what does our mind interpret?

He will consider that the small object is the “figure” and the large one is the “background” (our perception organizes the scenes into “figures” and “backgrounds”, as pointed out by the Gestalt school).

And the heuristic that we will apply, that is, the favorite “default option”, is that the small object, the figure, is the one that moves, not the background.

This will be true in the vast majority of the time (that’s why evolution has favored us to have that heuristic!) But sometimes it is wrong and that is where the illusion of “induced movement” occurs.

This illusion was studied by the psychologist Karl Duncker and consists of attributing to an object the movement that actually belongs to another. A powerful and fun example can be seen in this video, finalist in the “Best Illusion Year Contest”, entitled Once upon a time. The video itself will explain what happens, we’ve done enough! spoilers!

We blame the moon for our movement

Returning to our case, due to the effect of induced movement, a motionless moon next to a moving environment will seem to assume the movement. If the clouds cover it half as they move under the optical flow, the effect of the moon following us will be even more powerful.

Perhaps at some point in this article you have found that we create unnecessary problems, such as identifying objects (the moon, trees …) in a scene, deciding which ones move and which ones don’t, knowing how we move with respect to the environment …

But these things we do without thinking require a lot of work: It is estimated that visual processes can consume half of the brain’s resources. The technological field of “machine vision” strives to identify and mimic those “heuristics” that our minds use to make sense, seemingly effortlessly, to the images we see. Perhaps one day we will have machines capable of experiencing the moon following them!