I agree that Zhang Heng and Shen Kuo are both wrong. As any mathematician will tell you, all depends on the frame of reference. What is perpendicular to me is slanted to you since you and I stand in different positions.
But I also must disagree with your statement that the moon appears full "when the Sun rays are perpendicular on the Moon's surface". Draw a line from the moon to the sun. The moon appears full to us when the angle between the earth and that line is equal to the angle between the sun and that line. Per Euclid.
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