Uranus is actually visible to the naked eye under a dark sky:
At magnitude 5.8, Uranus is certainly naked-eye visible to observers with good vision and access to a dark site, but it is best observed in binoculars, in which its subtle aqua-green color becomes apparent and its lack of twinkle helps to distinguish itself from the background stars.
Suppose a sharp-eyed astrologer discovers Uranus (a difficult task given the number of stars of this magnitude, but not impossible) around 1500 BC. He follows the planet for a while and notices it behaves like all the other "wanderers".
What happens now? 8 day week? Improved optics to try to see Uranus (and possibly discovery of Neptune?). Does the Sabbath occur every 8 days? Improved star charts helping to discover longitude earlier?
Let's say the discovery takes place in Egypt by a temple astrologer who is able to go into the desert to get away from the city lights. The Mesopotamian empires learn about this and confirm the observation.