The visual perception of the targets at sea is the most inlportant factor for safety and for a good lookout in preventing maritime traffic accidents . This study shows the conditions for being able to perceive the visible horizon between the sky and sea surface and this. Author shows the visual and nautical conditions thereof ; they are as follows ; 1. The conditions of visual perception of the line of division between the sky and sea surface is that the luminance difference of the sky ( about

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... degrees over horizon )and sea surface ( about 2 degrees under horizon )is more than the Iuminance difference threshold . 2 . When nobody can perceive the above line of division ( horizon) , the visual and nautical environ − ment is indicated as fo110ws ; 〔 1) The luminance of the sky ( about 2 degrees over horizon )is O. 3 cd / m : . ( 2) The luminance of the sea surface ( about 2 degrees under horizon )is O. 03 cd / m : . ( 3> The difference luminance threshold is O. 27 cd / Ms . But in the case of O degree which is a relative direction towards the direction of the sun . 3.Asun altitude with above the mentioned condition is − 6 − − 9 degrees ( 1/ 3 -1/2 dtエration of astronomical twilight), which is connected with the relative direction towards the direction of the sun . 4. The lum 正 nance of the sky and sea surface and the illuminance of the sky have gone up and down with the sun ' s altitude , and they change rapidly and widely with the case of the time period from sunset and sunrise to − 9 degrees of the sun ' s altitude .