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.