Slashdot Mirror
Low-Hanging Moon Explained
gollum123 wrote to mention a BBC article which explains the low-hanging moon of the past few nights. From the article:"For the past few nights the moon has appeared larger than many people have seen it for almost 20 years. It is the world's largest optical illusion, and one of its most enduring mysteries. The mystery of the Moon Illusion, witnessed by millions of people this week, has puzzled great thinkers for centuries. There is still no agreed on explaination for why the moon appears bigger when it's on the horizon than when it's high in the night sky."
I've heard however that the illusion even occurs while flying high in an airplane. A horizon of clouds really doesn't give much of a landmark to compare to.
The way I've heard it, humans subconsciously model the sky as a flattened dome. Thus, when presented with two objects of equal apparent size, one on the horizon and one at the zenith, the one on the horizon looks bigger (i.e. is perceived as having a larger actual size) because it's "farther away" than, yet appears to be just as big as, the object that is directly overhead (and thus "closer").
Step 1: look at the moon near the horizon
Step 2: now, block out the horizon and all other objects with your hands, and look at the moon
The moon looks MUCH smaller whe you frame it with your hands and block out the extraneous stuff.
Also works with the sun, etc.
I am an astronomer... and you appear to be mostly right. Which of course means that I'm about to nitpick.
First, "Consequently, the Moon appears in its most southerly position, and it appears to 'hang' lower in the sky than during winter months for viewers in the Northern hemisphere (this effect is reversed for Southern hemisphere viewers)."
It's true that the seasons move the location of the ecliptic (the Sun's annual path across the sky) and thus the Moon at night is further south when the Sun is further north. However, there's another effect at play here: the Moon has an inclined orbit (relative to the ecliptic). So depending on where you are in that cycle (it's 17.5 years long, if I recall right), the Moon's position above or below the ecliptic adds to or subtracts from the ecliptics north-south changes.
So it's not so much the timing relative to the solstice (the odds of the solstice being on a day with an effectively-full moon are at least about 1/9, after all), it's about the precession of the lunar nodes.
Also, the Moon is squashed near the horizon, not stretched tall. I have a great photo of this somewhere, but I seem to have lost it in my last move.