I might be wrong but I've always seen it as a 3.5" floppy disk with the casing removed and the disk part replaced by the Sun. So it's like the Sun with a 3.5" floppy shutter on the front. (The animated versions of the logo spin like a floppy would, too.)
It helps if you've taken apart a 3.5" floppy before. 
These days it should probably be a HDD arm or something instead of the shutter.
