A Matter of Perspective

This Tuesday is my favorite day of the month. It happens to be election day, but while all the political hopefuls are waiting for polls to close and for votes to be tallied, I await the appearance of a large orange ball over the eastern horizon; Tuesday is full moon. But there is one curiosity about this event that often provokes controversy. It involves one of the ways in which nature can play tricks on us.

I have always been a moon watcher, and apparently there are many of my kind out there. With or without a telescope, the moon is mystifying and beautiful. It is our closest neighbor in space, and the only other world, so far, that we have walked upon.

The full moon always rises in the east at sunset. There, it is exactly opposite the sun in the sky and is fully illuminated. One of the more curious things about moon watching, is how much larger the full moon appears when it rises. Most people who have observed moonrises have noticed it, and many have asked why this is so.

It might seem logical to assume the moon appears larger when it is full because it is closer to Earth at this time. The average distance of the Moon from the Earth is 384,400 km, (240,000 miles). There is a variation of 42,000 km, (26,250 miles), in its orbit from when it is closest to the Earth and when it is farthest from Earth. This may seem like a large difference, but the moon isn’t necessarily closer when it is full.

Another frequent guess as to why the full moon appears larger when it rises suggests that the moon’s image is magnified by the atmosphere. It is true that when the sun or moon are on the horizon, their light must pass through a thicker layer of atmosphere that can change the color we see. So it seems logical that the atmosphere might also play a trick on our eyes, making the full moon appear larger because of magnification, but that’s not the answer either. Indeed a trick is being played on our eyes, but it is not the atmosphere, it is something called the Ponzo Effect or Ponzo’s Illusion.

The Ponzo Effect has to do with the way our eyes judge distances and sizes. Our brain and eyes work together to interpret what we see. If we look outside and see a child and a house, and they both appear the same size, we instantly interpret this as the house being much farther away than the child. We don’t consciously think about this process, we just do it, and it usually works pretty well.

When we look at a rising full moon near the horizon, we see a similar effect. When the full moon is close to the horizon, our eyes and brain then have objects such as trees or buildings to compare the moon with. When the full moon is high in the sky, our brain has nothing to compare it with except maybe stars, and they are just points of light. For whatever reason, when we have something to compare the moon with near the horizon, it appears larger.

This is an optical illusion that wasn’t explained until the early part of this century. It is named after the Italian psychologist who figured out this phenomenon, Mario Ponzo. Ponzo, who lived from 1882 to 1960, concluded that the apparent difference was purely psychological, that the actual size of the moon at the horizon is the same as when it is overhead. It only seems larger at the horizon because our brains are playing a trick on us.