It has been frequently reported that a toilet drains clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. This has been perpetuated by its frequent appearance in news such as PBS and NPr as well as in classrooms around the country. It has become so prominent because of its resonance with the everyday; it is very pleasing to think that great cosmic forces control every aspect of one’s life.
This myth is based on the Coriolis effect, a phenomena that is caused by the rotation of the Earth. “Imagine a child on one side of a merry-go-round throwing a ball to a child on the other. Because of the carousel's rotation, the ball appears to be deflected away from the straight-line path in which it was thrown” (Esser ¶ 2). This metaphorically represents the Coriolis effect; the merry-go-round represents the earth, and the ball represents an object affected by the Earth’s rotation. this is a case of relative motion; the object is moving in a straight line but the earth is not, thus the object moves relative to the surface of the earth. The direction of this rotation depends on the position of the object relative to the earth; an object will be deflected clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
Mathematics tells us that Coriolis forces will have an insignificant effect on the direction of drainage. This is supported by many credible scientific sources, such as http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html, which dispels many existing myths and cites specific example of inaccuracies, and http://curious.astro.cornell.edu/question.php?number=516, which presents the viewpoint of those familiar with such cosmic forces. Despite the inaccuracy of this myth, it continues to perpetuate itself through inaccurate information, and thus will not likely be dispelled in the foreseeable future.