[Note: This comic has been altered for clarity. I felt the “shallow bowl” explanation works better without the confusion of comparing the moon to clouds. You can view the previous version of the comic here.]
For reference, take a look at this comic…
The other day I received a surprise letter from none other than legendary star blogger Phil Plait, who wrote to correct me on a comic linked above (the first of the NDT comics, no less). It was a friendly “Oh, you knuckleheads” and a noogie of an email informing me that while the big-moon-on-the-horizon illusion is real, the mechanism behind the apparent size differences of the moon is not due to the oft cited “foreground comparison.” In fact, the reality is way more interesting.
In a fascinating article Phil wrote in 2010, he explains that it is not the moon in relation to buildings that makes it big, but the moon in relation to perspective in general. This is known as the Ponzo Illusion, and I’m almost 100% certain that you’ve seen it before. It took me some time to wrap my head around how exactly the two illusions are related, but after drawing the comic I can now not only understand the effect, but I think I also have an idea of how it may have been misconstrued into the explanation that Neil Tyson gave in the other comic. Do note that I had taken Tyson’s dialogue straight from a tweet, so yeah. It’s also quite possible that I missed a correction tweet down the line. That said, I’m secretly hoping for some astronomical mud wrasslin’ come NECSS 2012.
If you read the comic above, you can imagine how my demonstration with the clouds may be misinterpreted to mean that the moon looks big because of the small clouds. Which is false. The moon would look large even on a flattened horizon with no reference objects. In fact, it works for any celestial body. For example, Phil mentions that he witnessed a gigantic Orion rising over the horizon. A breathtaking sight for sure!
Thanks again Phil! I hope you enjoy your portrayal.
Update: I had an interesting conversation regarding why the Ponzo illusion still works in three dimensions, rather than just on paper where there is no depth of field. Definitely something to look in to. Immediately, this scene from Willy Wonka and the Chocolate Factory came to mind.