### Why does the Moon appear to follow us?

You know when you are in the car at night and you look at the Moon… and it seems that everywhere you go, the Moon goes too? Why is that? Three-year-old Lily Mae (from Dublin) noticed this recently so her Mum sent me in the question!

Distance and angles

When we travel in the car we notice all the objects like trees, buildings, even people, whizzing past our window. But if we look up at the Moon it doesn’t whizz past at all. In fact it seems to follow the journey with us. Did you ever wonder why? The short answer is that it is all down to how close the object is to our moving car. The trees, buildings and people are all fairly close to us, so as we pass by the distance we observe them from, changes very rapidly. The angle we observe them at changes too.

The Moon however, is very far away, 384,400 km away, to be precise. Even though our car is moving, it is not moving any closer, or farther away from the Moon, so the distance we observe the Moon from stays the same. As we look out the car window the Moon is still there, as if it is following us.

The distance between the Moon and the Earth is so great that the distance we travel by car is very small in comparison. The angle we observe the Moon from is not altered by the distance we travel in the car, for the same reason.

Test it yourself

Next time you are travelling in a car you can check out this theory with a little experiment; first, pick an object that is close to you and see how quickly it passes by your car window.

You can even time it, see how many seconds it takes. Now pick an object that is much further away from you … look to the horizon if you can, find a distant building, or even better a mountain or some other large object. Time how long it takes for it to pass your window. Did you notice a big difference?

Optical Illusion

This observation that the Moon appears to follow us as we travel is referred to as an optical illusion. The fact that the Moon is not near any objects that will allow us judge its distance, is a point that adds to the illusion.

We observe another optical illusion involving the Moon when it rises and sets. It is called the Moon illusion. To our eyes the Moon appears fuller when we see it rising or setting.

We think it looks smaller when it is high in the sky. Of course the Moon does not change in size, it is just a trick of the mind. When we see the Moon near the horizon, we are comparing it in size to another object but when it is high in the sky our eyes have nothing to compare it to.

If you want proof you can try another simple experiment. When you see the Moon rising, roll up a piece of paper and look at the Moon through it. Make your tube of paper a little wider than the Moon and then tape it in place.

Look at the Moon again a few hours later, when it is high in the sky; you will see that your tube of paper is still just a little wider than the Moon.

Naomi is a science communicator and mother to three inquisitive children. She can be found at sciencewows.ie

If your child has a question email: drhowsciencewows@gmail.com

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