Since everyone else here was putting up banners, I created a banner for my blog. The text and figure come from Chapter 8 (The Deltoid) of EH Lockwood's A Book of Curves (1961). It was on sale for $1 at a book sale, so I snapped it up.
A deltoid is the concave triangular curve formed when a small circle rolls around the inside of a circle three times as big. Eric Weisstein's Mathworld has a nice animation as well as a description of its properties.
If you've ever played with Spirograph you might have drawn a deltoid. Under the fold is a spirograph applet (courtesy of Anu Garg) that lets you create this curve and other pretty ones:
I always assumed that deltoid was a reference to the awe-inspiring shoulders you developed from years of power-lifting.... Next you'll be telling be that my subscription to Penthouse is not for the real estate listings!
The interesting thing is how a simple process can produce spikes. The entire economics profession is based on everything being non-spiky. In other words variables like tax and consumption are linked by smooth curves with no abrupt changes.
It's good to see the deltoid again - although I misss the lime green.