This post is for you, kids (well and parents too). You know what I am talking about, right? When in the car, it looks like the Moon is following you….and maybe it is. Well, here is a video showing something similar. You can’t see the moon, but it looks like the clouds are following us (I got my daughter to make this movie – just so you know I was being safe).
So, what is the answer? Why does the moon follow you? First, the simplest answer (then I will give a more complex answer). The moon appears to follow you because it is so far away. When the car moves 100 feet forward, the relative angle between the direction of motion and the car is essentially the same. This means that the moon appears in basically the same angular position with the same angular size. That normally happens when something close is following you (flying in formation as your wing man).
Ok, that was a little complicated. It is diagram time.
To make things simpler, I have a very close moon starting at a location perpendicular to the direction of motion (the gray boxes is the car at two different times and locations). The closer circles are trees near the road (the orange one started having its leaves turn colors). So, by how much does the angle of the object change? Basic right-triangle stuff says:
Solving for the change in the angle (θ) as seen from the car:
Now for some values. Suppose that I am driving 70 mph (about 31 m/s). In 1 second, the car will have moved 31 meters. If the distance to the moon is 4 x 108 meters, then the angular position (as seen from the car) will change by 4 x 10-6 degrees. Or in layman’s terms, not a heck of a lot. From the person in the car’s viewpoint, the moon is in the same position.
What about a tree that is just 10 meters away? In one second that angular position will change by 72 degrees. This is what makes it look like these trees are moving and the moon is not.
Just to take this one step further, I made another video.
Hopefully you can pick out the question-asker way in the back. Notice how it looks like he is following the camera? Oh, he is far away. Well, you can tell he is far away because of all the stuff in between the camera and the child. With the moon, there is no visual cues to indicate how far away it is. This just adds to the effect that it is following you.Go Back to Top. Skip To: Start of Article.