Which movement of some planets can NOT be explained in Plato’s system? 
(A)Backward 
(B)Forward 
(C)Upward 
(D)Downward

It is easy to understand why the Greeks found geocentric地球中心的 cosmology宇宙學 so appealing吸引力.   Night after night每晚, we see celestial天的 objects moving across the sky from east to west.

Wouldn’t it be natural to assume假定 that the Earth lies motionless不動 at the center of the universe and that the Sun, the moon, planets, and stars evolve旋轉 around it?

In the 4th century B.C., Plato柏拉圖 postulated假定，假設 a world system with the stationary靜止的；不變 Earth as its hub中心 and a huge outer space that carried the planets and stars in its daily revolution革命.

But his two-sphere圓形物體, 球 configuration佈局；構造 could not account for（在數量上）佔 a particular motion of certain planets.

As a rule, the planets and stars traveled together across the sky every night from east to west.

But now and then, inexplicable無法說明, some planets seemed to drift（尤指在外力作用下）飄 backwards. Today we know that retrograde倒退的；退步 motion is an illusion幻覺，幻想 caused by the fact that we observe planetary motion from a planet that is itself in motion.

It never entered the mind of Eudoxus欧多克索斯, a young contemporary存在於現在的, 當代 of Plato’s, that the Earth could actually move.

But he did try to accommodate供给, 使适应 planetary行星 retrogression倒退,退化 to the motionless earth by developing from the two-sphere圓形物體, 球 Platonic（戀愛或感情）純精神而無性愛的，柏拉圖式 system a model with twenty-seven spheres圓形物體, 球, one for every known planet; each was attached to another sphere whose rotation（尤指圍繞一個固定點）旋轉, combined with the rotation（尤指圍繞一個固定點）旋轉 of the other planetary spheres, explained the “reverse（使）反向；（使）倒轉” direction方向 of certain heavenly天堂 bodies.

But now and then, in...

It is easy to understand why the Greeks found geocentric cosmology so appealing. Night after night, we see celestial objects moving across the sky from east to west. Wouldn’t it be natural to assume that the Earth lies motionless at the center of the universe and that the Sun, the moon, planets, and stars revolve around it? In the 4th century B.C., Plato postulated a world system with the stationary Earth as its hub and a huge outer space that carried the planets and stars in its daily revolution. But his two-sphere configuration could not account for a particular motion of certain planets. As a rule, the planets and stars traveled together across the sky every night from east to west. But now and then, inexplicably, some planets seemed to drift backwards. Today we know that retrograde motion is an illusion caused by the fact that we observe planetary motion from a planet that is itself in motion. It never entered the mind of Eudoxus, a young contemporary of Plato’s, that the Earth could actually move. But he did try to accommodate planetary retrogression to the motionless earth by developing from the two-sphere Platonic system a model with twenty-seven spheres, one for every known planet; each was attached to another sphere whose rotation, combined with the rotation of the other planetary spheres, explained the “reversed” direction of certain heavenly bodies. 46 According to Greek cosmology, which of the following did NOT move?