Photo: Tahir Hashmi (Flickr)
Do you remember in my last post when I wrote that a Harvest Moon isn’t larger than any other moon? Well it isn’t, and neither is the Hunter’s Moon of October. Nor is the moon necessarily at a perigee – positions in its elliptical orbit closest to the Earth – during an equinox.
Notice that the moon really only looks particularly large when it’s close to the horizon. This large moon is actually an illusion – it is called the moon illusion. Anytime a celestial body is near the horizon it will look bigger to us, consider that the sun also looks larger to us when it is close to the horizon.
Constellations will also appear larger, but this is a bit harder to notice unless you are paying close attention to the changing night sky.
An Ancient Mystery
The moon illusion has been observed for thousands of years and theories abound. There have even been two books published on the subject! The most recent and accessibly written book is The Mystery of the Moon Illusion (2002).
The moon illusion is a well-debated but not a well-understood phenomenon. The theories are many and varied. The astronomer Claudius Ptolemy wrote in the 2nd century CE that the effect was the result of Earth’s moist atmosphere magnifying the moon – just as objects under water appear larger (refraction).
Early scientists eventually decided that the apparent size of the low-hanging moon was not an astronomical phenomenon, and from the mid-17th century CE onwards scientists have focused on the effect as an optical illusion – a perception of something that is different than objective reality.
So What Is The Moon Illusion?
There are lots of reasons why our brains may perceive the sizes of objects as they do, so there is still not absolute consensus on what causes the moon illusion. However, there is one theory that is quite popular among scientists.
This theory combines two effects: something called the Ponzo Illusion and our perception of the shape of the sky.
The Ponzo Illusion is the illusion that objects that are further away are larger. This coupled with the fact that we view the horizon as much further away than the sky’s zenith causes objects on the horizon to appear larger to us.
So even though the moon on the horizon is only a little bit further from us than the moon at the sky’s zenith, we see the moon on the horizon as much larger than the moon at the zenith.
For examples of the Ponzo Illusion and a more detailed explanation of this moon illusion theory, see Phil Plait’s excellent post over at Bad Astronomy.
The Moon Illusion is a 1989 collection of essays about the effect (Google books)
Explaining The Moon Illusion (Proceedings Of The National Academy Of Sciences)
Solstice Moon Illusion (NASA Science News)