Who discovered the retrograde motion of the planets?

Answer 1

Retrograde motion is apparent (obvious) in the night skies and even prehistoric people were able to observe this "irregularity". Retrograde motion is documented even in prehistoric-dated carvings, in caves. This motion was a natural phenomenon to them, anyway, rather than an irregularity.

The "evening star" is followed pretty fast (in terms of days) by a "morning star". That's Venus what looks like during its retrograde motion. It's very typical and it happens for long periods of time (in terms of years) during approximately the same solar-calendar day of a season, every 8 years (the "ninth-year" cycle). Setting an example, Venus is going (to be) retrograde in the early spring in (the 21st century: 2001, 2009,) 2017, 2025, 2033, 2041, 2049,... That's inevitable and plain to see, for earth-dwellers! Venus is going to be retrograde in late autumn this year (2010) and this will happen again in 2018, 2026, 2034, 2042, and so on ...

Philosophers and scientists at a later time tried to explain and decipher this phenomenon and proposed some models, like the Epicycles (proposed by Ptolemy). At last, the introduction of the telescope unclouded this mystery, anyway. (Believe me, it has been an experience for me to look for the first time at the four major satellites of Jupiter through a telescope. No human unaided eye can be thus penetrating (magnifying) like a telescope does.)