Explain the term "variation" in relation to navigation.

In navigation, "variation" refers specifically to the angular difference between magnetic north and true north. This distinction is crucial for mariners and other navigators because compasses point toward magnetic north, which does not always align with true north (the geographic North Pole). Each geographic location has a specific variation, which can be expressed in degrees east or west, depending on whether magnetic north is to the east or west of true north at that location.

Understanding variation is essential for accurate navigation. When plotting a course or determining a vessel's heading, navigators must take into account this difference to ensure that they are following the intended route. Failing to account for variation could lead to significant navigational errors, particularly over long distances where small differences can compound.

In contrast, the other choices relate to different concepts in navigation. The difference between nautical and statutory miles pertains to units of measurement, compass readings at various times of the day involve changes due to magnetic declination, and water temperature variance is more related to environmental considerations rather than directional navigation.