"Knowing the sky" involves following the changes of the seasons and weather: The objects in the sphere of nature cannot avoid producing different manifestations according to these changes and the bird artist should thus observe the different colorings and shapes that occur in birds and flowers through the four seasons.
知天，意指隨著時令與氣候變化，自然界的物體不免隨之產生不同現象，畫鳥人就應觀察到鳥類、花卉在四季不同的色澤與形態。