I found myself marveling at the rising full moon over Portland last Saturday night. It certainly seemed larger than usual. My friend says this effect is caused by light refracting through the cool autumn air.
Is he right?
You're close. Oregon moons are about 20 percent larger than the national average this year. This is caused by a combination of high CO2 levels, reflections off snow and ice on Mount Hood, and, of course, congressional redistricting following the 2010 census.
Psych! I spin this line of bullshit not to mess with you—OK, maybe partly to mess with you—but also to remind you not to believe everything you hear.
Your friend, to put it charitably, is full of crap. Actually, I guess that's not very charitable. He's full of crap, but he has nice eyes? The point is that the size-distorting effect is caused not by the air but by humanity's stupid brain.
When we see the moon on the horizon—with trees, houses and hills for scale—it seems far away, so we process it as bigger. When it's overhead, though, our puny minds can't grok the distance. We subconsciously assume the high moon—and the flat, dishlike sky it's so clearly glued to—is, oh, 200 or 300 yards away, and hence, not that big.
If your friend doesn't buy this (not that anybody would ever disagree with Dr. Know), tell him to compare the next big, low moon he sees with the size of his thumbnail at arm's length. Then have him try the same trick when the moon is high in the sky. It'll be the same size.
In the meantime, if he tries to convince you that you can't get pregnant with your shoes on—well, don't say I didn't warn you.