A couple of weeks ago I posted regarding the changing size of the Moon as seen from the Earth due to its elliptical orbit. The question was raised initially due to a conversation a friend of mine had with someone who thought that a large Moon was a harvest moon and occurred at harvest time each year.
Tomorrow will in fact be the harvest Moon for 2006. This is always the Full Moon which lies closest to the Autumnal Equinox and does indeed mark the time for harvest to be brought it. However it is not necesarily bigger or smaller than any other Full Moon through the year.
I thought though that it may be worth posting regarding something else to do with the Moon’s size and that is the ‘Moon Illusion‘ or ‘Moon Effect‘. This is the effect that we have all no doubt seen at some point, where the Moon appears huge on the horizon. You might well notic eit tomorrow if the weather is clear.
In truth the Moon is not bigger at all, and it is a common (but incorrect) belief that it looks larger due to the Earth’s atmosphere magnifying the image along the line of sight. What is really happening is far more confusing and complicated and in fact: unknown.
The odd part is that the Moon Illusion is easily proven as an optical illusion. Next time you see what you think of as a ‘large’ moon near the horizon then hold a penny out at arm’s length and compared how big the Moon looks compared to it. Then after a few hours go back outside, hold up the same penny and compare the now higher up Moon to it again. The two will remain at the same relative size - that is to say that you will realise that the Moon is no bigger or smaller than it was earlier in the night.
Yet your eyes seem to be telling you a different story. Or in fact it might just be your brain that is lying to you. Let’s take the example below. The two arcs are the same size but most people see them the top one as being shorter.
This effect is called the Ponzo Effect and is also shown is the following illustration. Only here the converging lines cause the two identical circles to appear to be of different sizes.
Some people propose that the effect of other objects in the field of vision, such as stars, trees, clouds etc are affecting the percieved size of the Moon’s disc in our mind. The idea is disputed somewhat though, since stars are often not visible around a bright Full Moon and there need not be any trees, clouds, houses or other objects present for the Moon to seem large.
A slightly better explanation may by the idea of the Sky Dome. Now I should stress at this point that this theory hasd its problems too. In fact any currently proffered explanation of the Moon Illusion has some kind of issue with it. There is no difinitive correct answer as yet. The Sky Dome is simply one example and for more you can check out this web link.
The Sky Dome model simply suggests that our mind changes the real shape of the sky since it cannot stereoscopically discern the location of objects. The sky is essentially a plane arcing over our hears and this theory proposes that our mind doesn’t quite get the arc right as it analyses the objects dranw upon it.
It is best explained by invoking the above diagram which shows the actual arc of the sky above an observer and the percieved arc created by our eyes and brain. As you can see, the angles of objects directly above our heads are projected onto a smaller area and thus appear smaller than objects at the horizon.
Don’t worry if you can’t get your head around the problem of a large Moon - humanity has been puzzling over itsince the seventh centruy BC. Ancient Greek and Chinese astronomers couldn’t fathom it, nor could Aristotle. But next time you see a big Moon at the horizon and someone tells you that its due to the atmosphere magnifying it - ignore them. Or even better: correct them…
