My friend Dave is an astronomer and was intrigued by mechanical Orrery’s and especially one that could be 3D printed. This one was found on the internet. The idler gears in the original design were two separate gears with a pin to connect them. The pin was made as part of the print on one gear. The pin is a crossed boss into crossed slots on the other gear. Various other tweaks for smooth operation. 

Parts were printed in PLA on various Ender3’s.