The word planet was meaning moving star. Every people who have access to a sky without light pollution will notice the moving star aka the very obvious one : the sun and the moon. There is the obvious one : Venus which is the brightest start and always at the end or the beginning of the night, Jupiter which is a bright yellow dot and Mars a red one. There is also Mercury and Saturn which are a bit more tricky than the other to spot with eye only. This is the origin of our 7 day week which came from the Babylonian. At least in french each day of the week is linked to a planet : Lundi - Moon (Lune) Mardi - Mars (Mars) Mercredi - Mercury (Mercure) Jeudi - Jupiter (Jupiter) Vendredi - Venus (Venus) Samedi - Saturn (Saturne) Dimanche - Sun (Le soleil and also from the latin Dominus wich means the lord)

The other planet were discover using the gravitational law from Newton. First you have to buid a file with the position of the planet durring many years and with a good precision. To achieve such a precision clock and Astrolabe were necessary. Then you have to compare the predicted position with the measure to spot the anomaly. With good math and hypothesis you can guess a new planet. You use a telescope to check your hypothesis and you have discover a new planet.