'Cheon' is one of four categories with 'Ki'(氣), 'Do'(道), ‘Ri'(理). Specially 'Cheon' meaned sky is so important that most of philosophers have referred to it. 'Cheon' is widespread, so it has been the object of the astronomers and subject of the philosophers. All recognize the conception definitely and consistently, in the other hand some vaguely, someone give different conceptions. Therefore this paper is written with a view to right recognition of the conception and relationships with the human by the study of significance changes by the times.