Super Moon

Conversation 17

SOCRATES. Well friends, what’s on your minds this morning?

CRITOBULUS. This blue super moon we had last night, it didn’t look at all blue.

EUTHYDEMUS. And what was super about it?

S. What do you say, Adeimantus?

ADEIMANTUS. I think ‘blue’ and ‘super’ as applied to the moon are just popular labels loved by journalists. They have no astronomical significance. I can tell you when they are used, if you want me to.

S. Fire away, Adeimantus. Start with the blue moon.

A. The second full moon in any calendar month is popularly called a blue moon.

S. Is that so, Adeimantus? What was the source for your information?

A. Wikipedia, of course.

C. So, it has nothing to do with looking blue?

A. Apparently not, Critobulus.

S. When I were a lad, a blue moon symbolised rarity. It also had connotations of wistful, romantic melancholy, no doubt traceable to the great Rodgers and Hart ballad of 1934.

A. I bet you got that information from Wikipedia as well, Socrates.

S. What if I did, Adeimantus?

C. Is that the one that goes, ‘Da-dang-da-da-dang, da-dingy-dong-ding, Blue Moon?’

S. You refer to the Marcels doo-wop version of 1961, Critobulus, although I first heard it sung by the Delltones on Brian Henderson’s Bandstand TV show a year or two later.

C. You’re in danger of giving away your age, Socrates.

S. I assure you I was hardly born then. But Adeimantus is right; there is no astronomical significance to the blue moon. What about the super moon, Adeimantus?

A. When a full moon occurs at the time when the moon is near perigee, that is, at its closest point to Earth in its elliptical orbit, some people call it a super moon. It looks bigger and brighter because it is closer than usual.

C. But not at all blue?

A. Not usually.

E. So, there is some astronomical meaning to the super moon?

A. You could say that.

C. Well, that clears up the blue and the super, but I have another question. Why does the moon look bigger when it rises compared with how it looks when high in the sky?

S. Can anyone advance a theory to answer Critobulus’s question?

C. I bet you have a theory, Socrates.

S. I do, but I would like to hear what you others think.

E. My answer is that God sends a super moon when he thinks we need more light in the night.

S. No doubt God is happy to let the super moon occur, but since we can predict the occurrence of super moons with great accuracy, that would suggest we can see into the mind of God, which sounds somewhat disrespectful, Euthydemus.

C. I think it might be something to do with atmospheric distortion near the horizon, caused by more dust, or convection currents.

S. At least you have proposed a theory that’s testable by scientific means, Critobulus. Your postmodernist friends would not approve, I fear. They might rate your theory lower than the one Euthydemus has proposed, except that his theory referred to the Christian God, which is a no-no for postmodernists. I’m sure if the Hottentots had an explanation, the postmodernists would rate that higher.

A. Not that we have anything against the Hottentots, or their theories about the apparent size of the full moon.

S. Of course not. Admirable people, the Hottentots. But Critobulus, how do you know there is more dust or convection currents near the horizon.

C. I have observed that dust haze seems to be greatest close to the horizon and I think that convection currents are caused by the temperate differential between the air and the ground which has been heated by the sun. When we look at something near the horizon, we are looking through more dust and currents than when we look higher in the sky.

S. That’s not bad, Critobulus. Now tell us, how do you think dust or convection currents make the moon look bigger. Does everything else near the horizon look bigger as well? A house for example?

C. You never see a house high in the air, so how can you tell if it looks bigger near the horizon?

S. You are getting closer to the true answer than you realise, Critobulus.

A. I think it is an optical illusion, but I can’t explain it.

S. You are right that it is an illusion in the mind. If you use an instrument to measure the angular size of the moon, you find it is the same near the horizon as it is high in the sky. Try it yourself. Next time there is a full moon, take out your quadrant and measure the angular width of the moon when it is low in the sky and when it is high in the sky.

E. What if I don’t have a quadrant?

S. Take a photo of it near the horizon. Later, on the same night when it is high in the sky, take another photo with the same camera and settings. Compare the sizes of the two moon images. I am prepared to bet they are the same size.

C. Do we have to wait for the next blue moon before we can do the experiment?

S. Perhaps not. If I remember correctly, the next blue moon will occur in 2037. Any full moon will do, so you have less than a month to wait.

A. Are you going to tell us your theory, Socrates?

S. I believe Adeimantus is right, it is an illusion. This is my explanation for the illusion. It is only an hypothesis; I have not done any experiments to test it.

A. Tell us anyway.

S. Right! Imagine you are a primitive man camped out on the African savanna with your small clan.

C. Perhaps you are a Hottentot.

S. You might be, Critobulus, although I was thinking of someone in paleolithic times. You scan the horizon for approaching threats. What are you most worried about?

E. A tiger!

A. A tiger? In Africa? Hardly, Euthydemus.

C. Lions?

S. Lions were probably more likely than tigers, Critobulus, but I think there was a greater threat.

C. What could be a greater threat than lions?

S. Other men, Critobulus, coming to steal your food and women.

C. I suppose that could be right, but what has it to do with the size of the moon?

S. You see a man approaching among the trees in the distance. He is silhouetted against the horizon like the rising moon. What do you really want to know about him?

E. Is he bigger than me?

S. Fair enough, Euthydemus. Perhaps you also want to know how far away he is. Will you have time to run, or to gather your fellow fighters?

C. Yes, you need to know how far away he is.

S. Spot on, Critobulus. And how will you estimate how far away he is?

C. By how big he looks.

S. We are getting there. Our primitive man knows from much experience how big a man looks at various distances. He has a good feel for it. His brain, in fact, has been wired by evolution to make those estimates quite accurately.

A. Agreed, but we still have to make the connection with the moon.

S. The moon subtends an angle of one half of a degree. A man standing about 200 metres away also subtends an angle of about of one half of a degree. Do you see where I am heading with this?

C. Perhaps you could spell it out.

S. Imagine our primitive man sees the approaching man behind the first row of trees and looking over them, obviously more than 200 metres away. How does he look to our man?

A. He looks like an enormous giant. Our man drops his spear and runs for all he is worth.

S. No doubt, Adeimantus. He knows that the approaching man who subtends an angle of one half of a degree and is obviously more than 200 metres away must be enormous. Now I ask you, ‘How does this apply to the moon?’

A. I’m not sure. We have a good idea how big a man is, but we have no idea how big the moon really is.

S. That’s part of the problem, Adeimantus. If we were looking at a house, as we were saying earlier, we would have a pretty good idea how big it was and would be able to judge its distance reasonably accurately. But not so with the moon. Besides the distance, what else do we want to know about the approaching man?

C. Whether he’s friendly or nasty?

S. Yes, how will we judge that?

C. By the expression on his face?

S. Quite likely we will be interested in the expression on his face, Critobulus. What do we see when we look at the full moon?

A. I see where this is going, Socrates. We tend to see the moon as a face. We are programmed to see faces.

S. So we might imagine, probably subconsciously, that the moon is really the size of a man’s head.

C. I always think the fronts of cars look like faces. Some are happy and some are sad.

A. I’ve always thought car designers should pay more attention to the facial expressions of their cars. The angry or sad ones are quite off-putting, I find.

E. I’ve always thought the BMW grill looks like Adolf Hitler’s moustache. I wonder if that was intentional.

S. We are beginning to stray from our topic, although the point about our tendency to see faces and judge mood is quite relevant. It is built into the human mind. Now let me state my theory about why the full moon looks bigger when it is rising. We humans are primed by evolution to judge the distance of an approaching human by how big they look, particularly by the angle they subtend on the horizon. As well, if we think we know how far away they are, we estimate their size from the angle. We don’t know how big the moon really is, but we subconsciously feel it is a face, and so about the size of a man’s head. But it comes up behind the trees, so it must be quite far away. Therefore, we feel it must be enormous. It looks big.

A. Then why does it look smaller as it gets higher in the sky?

S. I admit I am guessing, Adeimantus, but I would say that when the moon is high in the sky, our subconscious no longer thinks of it as an approaching man. Also, there is nothing in the sky to judge the distance of the moon against. We don’t form a subjective impression of the moon’s size or distance. We just accept it as it looks, which happens to be smaller than our impression when it is rising.

C. I’m impressed, Socrates. You have obviously pondered this problem. How can we test your hypothesis?

S. I’m impressed too, Critobulus. You are starting to think less like a postmodernist and more like a scientist.