Their Earth-sized colony planet, Atlantis, is a moon of the massive gas giant planet, Minos. Thus, the Atlantean sky contains:
two suns, one merely the brightest star although visible by day;
Minos, permanently above the inhabited hemisphere, several times larger than the Moon as seen from Earth, with visible storms, daily eclipsing the nearer sun;
four other Minoan moons, two visibly moving.
Although without a moon of their own, Atlanteans see seven heavenly bodies apart from the fixed stars. This complicated celestial activity has planetary consequences. High, shifting tides make alternating salt marshes or lakes, to which local life has adapted, of the coastlines.
About a thousand women reproducing parthogenetically have generated a caste system based on the psychophysical aptitudes of the different genotypes. However, other social arrangements have also developed:
the veiled Doctors controlling the parthenogenesis machine in the ancestral Ship have become a neutral but powerful priesthood;
refugees colonizing a fertile, easily defended river island became anarchic and unwarlike;
salt marsh dwellers have degenerated to neolithic nakedness;
the freer, more mobile sea people have a democratic republic.
As ever, Anderson considers sociology as well as planetology.