light reflected from the Sun. Jing Fang had written in the 1st century BC of how it was long accepted in China that the Sun and Moon were spherical in shape ('like a crossbow bullet'), not flat. Shen Kuo also wrote of solar and lunar eclipses in this manner, yet expanded upon this to explain why the celestial bodies were spherical, going against the 'flat earth' theory for celestial bodies. However, there is no evidence to suggest that Shen Kuo supported a round earth theory, which was introduced into Chinese science by Matteo Ricci and Xu Guangqi in the 17th century. When the Director of the Astronomical Observatory asked Shen Kuo if the shapes of the Sun and Moon were round like balls or flat like fans, Shen Kuo explained that celestial bodies were spherical because of knowledge of waxing and waning of the Moon. Much like what Zhang Heng had said, Shen Kuo likened the Moon to a ball of silver, which does not produce light, but simply reflects light if provided from another source (the Sun). He explained that when the Sun's light is slanting, the Moon appears full. He then explained if one were to cover any sort of sphere with white powder, and then viewed from the side it would appear to be a crescent, hence he reasoned that celestial bodies were spherical. He also wrote that, although the Sun and Moon were in conjunction and opposition with each other once a month, this did not mean the Sun would be eclipsed every time their paths met, because of the small obliquity of their orbital paths. Shen is also known for his cosmological hypotheses in explaining the variations of planetary motions, including retrogradation. His colleague Wei Pu realized that the old calculation technique for the mean Sun was inaccurate compared to the apparent Sun, since the latter was ahead of it in the accelerated phase of motion, and behind it in the retarded phase. Shen's hypotheses were similar to the concept of the epicycle in the Greco-Roman tradition, only Shen compared the side-section of orbital paths of planets and variations of planetary speeds to points in the tips of a willow leaf. In a similar rudimentary physical analogy of celestial motions, as John B. Henderson describes it, Shen likened the relationship of the Moon's path to the ecliptic, the path of the Sun, "to the figure