So, I've put together a small theory that we had 11 planets. On this idea I came reading this page.
Summarized: the Egyptians revered the moon as the eye horus. This was initially visible every night (!) . In a battle between osiris and horus, osiris was killed. Seth shook his eye . Horus let the eye heal again and then sacrificed it to osiris again. Since that incident, the moon has disappeared again and again (constant alternation between sacrifice and restraint).
I didn't find the right solution from the Internet.