Why was retrograde motion important in changing people's ideas about the solar system?

Planets in the solar system orbit move eastward when orbiting the sun. retrograde motion is the apparent motion of a planet in a direction opposite to that of other bodies within the system, that is from east to west. for example, viewed from earth, there are times when mars appears to be traveling backward for a short period of time, i.e. from east west. this is because of the relative motion of earth; as earth passes the slower moving planets e.g. mars and jupiter, even though the planets are moving eastward but from the earth, they appear to be moving westward. this was explained by the heliocentric model of our solar system, with the sun at the center of the solar system and other planets orbiting it. hence the observation of retrograde motion was important in changing people's ideas about the solar system as a geocentric model to a heliocentric one.

Where is the diagram? you need to be more specific in your question so we can you