How come that in the world of the Third Horizon, Al-Ardha is a long-lost memory?
There were several hundred thousand people from Terra in cryosleep on the Zenith.
Did they lose their memories during their thousand-year long slumber?
If not, they should have had relatively recent memories of Earth, the planet of their youth.
Some of those might even be around in the present day.
Also, there should be some written records left.. people had a lot of time to write down their stories after they woke up.
Just some food for thought.