section_path: Scholarly achievements/Astronomy and instruments
section_name: Astronomy and instruments
paragraph_text: Astronomy and instruments
  
  Being the head official for the Bureau of Astronomy, Shen Kuo was an avid scholar of medieval astronomy, and improved the designs of several astronomical instruments. Shen is credited with making improved designs of the gnomon, armillary sphere, and clepsydra clock. For the clepsydra he designed a new overflow-tank type, and argued for a more efficient higher-order interpolation instead of linear interpolation in calibrating the measure of time. Improving the 5th century model of the astronomical sighting tube, Shen Kuo widened its diameter so that the new calibration could observe the pole star indefinitely. This came about due to the position of the pole star shifting in position since the time of Zu Geng in the 5th century, hence Shen Kuo diligently observed the course of the pole star for three months, plotting the data of its course and coming to the conclusion that it had shifted slightly over three degrees. Apparently this astronomical finding had an impact upon the intellectual community in China at the time. Even Shen's political rival and contemporary astronomer Su Song featured Shen's corrected position of the pole star (halfway between Tian shu, at −350 degrees, and the current Polaris) in the fourth star map of his celestial atlas.
  
  The astronomical phenomena of the solar eclipse and lunar eclipse had been observed in the 4th century BC by astronomers Gan De and Shi Shen; the latter gave instructions on predicting the eclipses based on the relative position of the Moon to the Sun. The philosopher Wang Chong argued against the 'radiating influence' theory of Jing Fang 's writing in the 1st century BC and that of Zhang Heng (78–139); the latter two correctly hypothesized that the brightness of the Moon was merely 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 of a rope coiled about a tree."
  
  Along with his colleague Wei Pu in the Bureau of Astronomy, Shen Kuo planned to plot out the exact coordinates of planetary and lunar movements by recording their astronomical observations three times a night for a continuum of five years. The Song astronomers of Shen's day still retained the lunar theory and coordinates of the earlier Yi Xing, which after 350 years had devolved into a state of considerable error. Shen criticized earlier Chinese astronomers for failing to describe celestial movement in spatial terms, yet he did not attempt to provide any reasoning for the motive power of the planets or other celestial movements. Shen and Wei began astronomical observations for the Moon and planets by plotting their locations three times a night for what should have been five successive years. The officials and astronomers at court were deeply opposed to Wei and Shen's work, offended by their insistence that the coordinates of the renowned Yi Xing were inaccurate. They also slandered Wei Pu, out of resentment that a commoner had expertise exceeding theirs. When Wei and Shen made a public demonstration using the gnomon to prove the doubtful wrong, the other ministers reluctantly agreed to correct the lunar and solar errors. Despite this success, they eventually dismissed Wei and Shen's tables of planetary motions. Therefore, only the worst and most obvious planetary errors were corrected, and many inaccuracies remained.