Which condition must be true for proper celestial navigation?

For proper celestial navigation, clear visibility of the celestial body is essential. This visibility ensures that the navigator can accurately identify and measure the celestial object, which is crucial for fixing a position at sea. The ability to see the celestial body clearly allows for precise observations and calculations, which are fundamental to determining latitude and longitude.

Other factors like verifying species of stars or specific angles of the sun are not strictly necessary for celestial navigation; while they can enhance the accuracy or provide additional context, they do not directly impact the capability to navigate by the stars or sun itself. The observer's stationary position is also not a strict requirement, as navigators can successfully use celestial navigation techniques even while in motion, provided they can maintain the necessary lines of sight to the celestial bodies. Nonetheless, having a clear line of sight is non-negotiable for the effectiveness of celestial navigation.