Before the planet Uranus was discovered, Capricorn shared its ruler Saturn with the sign Aquarius.
Saturn is still considered Aquarius' "sub-ruler", but it's principles manifest differently in the two signs.
Saturn is Chronos, father time, the one who makes us aware of the constrictions the physical realm holds - time, structure, lack, limitations, the scientific laws.
It's a very serious planet that looks at the world as a battleground.
The differences between the two Saturnian signs are their elements and modes.
Capricorn is the cardinal earth sign. It is both practical and achievement-oriented.
Capricorn fulfills the archetype of the father figure, the bread winner, the authoritarian.
They look at the world asking "How can I influence this planet on the biggest possible scale, be successful, earn respect, build up a good reputation?"
Aquarius is the fixed air sign. It is mental and humanitarian.
They use their minds to solve the problems of people - they see where the world is lacking in its rights and freedoms for people. Aquarius will always root for the underdog.
They look at the world asking "How can I influence society on the biggest possible scale, free people, destroy hierachies, use my right of freedom and self-expression?"