Answer:
Step-by-step explanation:
Because everyone understood that the mass of the earth exerted a force on the apple. It was what the apple did that was astonishing. The force between the apple and the earth implied that the apple exerted a force on the earth! We still have trouble with that idea.
Also who would have thought that sunlight was a composite of all colors. Who could ever imagine that white light has green or blue in it. Those three colors are so different.