USCG Celestial Navigation Practice Exam

Applying a phase correction is most relevant when observing planets. The reason for this is that planets do not exhibit a constant phase when viewed from Earth; instead, their apparent size and shape can change based on their position in relation to the Sun and Earth. This variability is due to aspects like the angle of sunlight striking the planet, which affects how much of the planet's surface is illuminated and visible from our perspective.

Correctly adjusting for this phase is vital to accurately determining the position and distance of a planet during celestial navigation. This adjustment enhances precision in the calculations necessary for navigation purposes. Other celestial bodies, such as stars and the Sun, do not require similar phase corrections for observations because they either appear as point sources of light or have a more constant brightness that does not change in a way that affects celestial navigation. Therefore, utilizing phase corrections is specifically applicable to planets to ensure the utmost accuracy in their positional data.