For anybody who wants to dig into the theory behind all this, these are examples of aliasing (the same word you see in "anti-aliasing" regarding fonts, computer graphics, etc.). I've been fascinated by moire patterns since I was a kid 45+ years ago. Here's one of my dabblings that required a bunch of browser-specific hacks to turn anti-aliasing off so the aliasing would show (if you change window size, you'll notice different effects after it redraws): http://www.nukacourt.com/moire.html