Myths
Does the Moon Turn Upside Down Below the Equator?
Does the Moon Turn Upside Down Below the Equator?
Myth Memo 1, 21 Sep 2001
Abstract. Two commonly held beliefs about the equator are (1) that the moon phase appears upside down below the equator—using my Northern Hemisphere bias—and (2) that water spins down the drain in the opposite direction there too. The brief answer is that neither of these is true. I will deal with the moon myth here. The fluid vortex myth is treated on the web in several places rather well. The moon myth is a little bit harder to dispel. In fact, myth is perhaps too strong a word as you will see, because there are many cases where the moon does flip over, or approximately so. It’s just not generally true.
And here is the conclusion: The moon phase in general does not turn upside down when changing hemispheres, even if one is rather sloppy about the meaning of upside down—say, 180° plus or minus 20°. However, there are clear situations where the moon does turn upside down. In particular, if one watches long enough during a day, he will generally see an exact upside down for a short while. It is perhaps these situations that have been reported and repeated until the idea has taken root as a general “fact.”
I provide an intuitive argument and then back it up with hard data from the astronomy program Starry Night Pro.
In case you are dying of curiosity, water goes down the drain in the direction of initial displacement, regardless of the hemisphere. There is a force, the Coriolis force, that in ideal circumstances, would force the vortex in opposite directions, but the ideal circumstances never occur except in very carefully controlled experiments—I think this care has been exercised only once or twice in history. Very large vortices are affected by this force. Hurricanes spin in opposite directions in the two hemispheres. A bathtub drain or a flushing toilet vortex are trivial in comparison and are, for all practical purposes unaffected by Coriolis. It is the direction the pipe is pointing, for example, that determines vortex direction, not the hemisphere. See, for example, http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html for further details. My thought exercise for this one is this: Imagine standing at the equator with a bucket of water. Start it spinning. Now, while it is spinning, step across the equatorial line. Do you really imagine that the water would stop, change directions, and start spinning the other way? Of course not. It’s just a line.