Final answer:
The Greeks found it difficult to explain the Parallax angle because they believed in an Earth-centered view of the universe, where the stars did not seem to show any significant shift in position. As a result, the concept of a moving Earth or the stars being extremely far away was beyond their understanding.
Step-by-step explanation:
The Parallax angle was difficult to explain for the Greeks because they believed that Earth was stationary and that all sky motions had to be circular. They made dedicated efforts to observe stellar parallax, but the brighter (and presumably nearer) stars did not seem to shift as they observed them in the spring and fall. This led them to retreat to the safety of the Earth-centered view, which dominated Western thinking for nearly two millennia.