Why does the moon appear to move alongside you?


You know, when you’re either walking, driving, or otherwise moving around at night the moon appears to be “following” you. I would assume it’s due to the scale/distance or something internally in our bodies related to coordination.

In: 4

You noticed how objects closer to you “move” faster when you go by due to perspective? The Moon is just so far away that it seems stationary. The Sun is even farther. The clouds are much closer but still pretty far away for this effect.

Because it’s very far away, and the further an object is away from you, the closer to parallel that lines of light have to travel to reach your eyes.

To demonstrate this, take a ball or something and put it on the table infront of you. Draw a dot on it. Get a ruler or an equivalent long straight thing and carefully position it so that it touches the dot and one of your eyes (or hovers just infront of your eye if you don’t want to touch it). Now take another long straight thing and do the same thing but for your other eye. Note that you have effectively made a triangle here between your two eyes and the dot. Here is an [image](https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2F2.bp.blogspot.com%2F-z_dj0qNdTPs%2FXBUgn08BzdI%2FAAAAAAAAXqA%2F0orq3G7C91oDrCufKNyCReiQz-m7TRoYACLcBGAs%2Fs1600%2FPicture1.png&f=1&nofb=1) of that triangle. Note that the closer to the base the peak of the triangle is, the greater the angle at the peak.

Now, imagine you extend that triangle upwards so that it’s really, really tall. The angle is going to keep shrinking, and the sides of the triangle are going to get steeper. If the triangle was so tall that it was the distance between the moon and your eyes, the sides would be so steep that to you they’d look like they were parallel. This is how our brains figure out how far things are away from us – the more parallel those lines of light, the further away it is. This also causes the image to change place less as you move, because the lines that hit your eyes barely move in relation to the object itself. The moon is so far away that these lines don’t appear to change at all, so the moon stays in the same place within the sky as a whole.

Essentially, it’s related to the insane distance of the moon.

When your face is pressed right up against your phone, it’s right in front of your face, and pretty close. When you move your face to the right, the phone is no longer right in front of your face. Instead, since you moved to the right about the same distance as the phone’s original distance from your face, the phone is now in front, and to the left of your face. It looks sort of like this:




A pretty noticeable change. On the other hand, the moon is insanely far away. Like, insanely far. So far it would take a month of driving towards it to reach it. So, when you start, the moon is in front of your face, like this:


So now, when you walk a few streets down, you’re barely moving relative to your starting distance. It looks sort of like this:


The moon is still essentially in front of you, not beside you. This janky demo is extremely exaggerated too. If this were to scale, you’d move less than a pixel.

In a more advanced way of saying it, the farther away something starts, the less a small change in position will affect its angle (and therefore where it appears in your vision)