Shen Kuo was the first to discern magnetic declination of true north while experimenting with a compass. Shen theorized that geographical climates gradually shifted over time. He created a theory of land formation involving concepts accepted in modern geomorphology. He performed optical experiments with camera obscura just decades after Ibn al-Haytham was the first to do so. He also improved the designs of astronomical instruments such as the widened astronomical sighting tube, which allowed Shen Kuo to fix the position of the pole star (which had shifted over centuries of time). Shen Kuo was also known for hydraulic clockworks, as he invented a new overflow-tank clepsydra which had more efficient higher-order interpolation instead of linear interpolation in calibrating the measure of time.