Final answer:
The reciprocal marks on a compass bezel are set at 180 degrees from the index marks and align with the lubber line to help navigate back to a starting point. They are not required by law but are crucial for effective navigation, especially when landmarks are not visible.
Step-by-step explanation:
The reciprocal marks on your compass's bezel are not required by law, but they are incredibly useful for navigation. These marks are generally set at 180 degrees from the index marks (not 90 degrees as mentioned in the question), which allows a navigator to easily determine a back bearing or reverse course. In essence, they help you to find your way back to your starting point without having to surface to take a new heading, which is critical in situations where visibility is poor or when traveling underwater, through dense forest, or in any environment where landmarks are scarce or indistinct.
These reciprocal marks are aligned with the lubber line, which is the fixed marker on the compass that indicates your forward heading when you take a bearing. By aligning the reciprocal mark with an object or destination, you can navigate directly to that spot. Once you want to return to your starting position, you simply turn the compass around so that the lubber line is now aligned with the reciprocal marks, which will guide you back along the exact opposite bearing.
It's essential to remember that declination or the difference between magnetic north and true north can affect compass readings, so declination adjustments should be made when performing precise navigation tasks. Additionally, the orientation of the compass needle will change as you move across the Earth's surface because the magnetic field lines, which the needle aligns with, vary depending on position relative to the magnetic poles.