The Moon & a Turtle

I was wondering if there was a Japanese expression equivalent to the English "like chalk and cheese" to describe two things that are completely unalike. It turns out there is: 月とすっぽん (the moon and a turtle). Superficially, the moon and a turtle are alike in that they are round in shape. However, the beautiful moon riding the night sky and the turtle scrabbling around in the mud below couldn't be more dissimilar. I wonder how this expression originated. Here's an example sentence from my copy of 2001 Japanese and English Idioms:

彼らは兄弟ですが、性格も趣味も月とすっぽんです。
Karera ha kyoudai desu ga, seikaku mo shumi mo tsuki to suppon desu.
Though they are brothers, their characters and tastes are as different as chalk and cheese.

Many thanks to Ueno-san of Tsurugi for teaching me this expression. And for those who are interested in the origins of the wonderfully alliterative "chalk and cheese", you can find some information here.