When converting true course to magnetic course, what do you do with easterly variation?

To convert true course to magnetic course, the key aspect to consider is how magnetic variation (also known as declination) affects the course. When dealing with easterly variation, you need to understand that it indicates that magnetic north is east of true north.

To calculate the magnetic course accurately, you must subtract the easterly variation from the true course. This is because the true north reference point needs to be adjusted back towards the magnetic north reference point, thus requiring a subtraction to align the true course correctly with the magnetic course.

In this context, recognizing the nature of easterly variation is essential: it represents a positive angle in terms of eastward displacement from true north, and therefore, to find the true course that aligns with magnetic heading, subtraction of this angle is necessary. This is why the answer is indeed to subtract when converting true course to magnetic course with easterly variation.