The stars are not visible by day, and the sky may be overcast at night. Sailing directions in the exercise are, at best, only to the nearest compass point. Conditions vary with the seasons. Application requires using what one can actually see, and it requires adjusting the sailing directions in the light of actual experience. Ocean swells are a crucial guide in sailing.
Navigators recognize up to eight different swells, one from each octant of the compass.
During the summer, swells come from the southeast and south. The different swells have characteristic intervals. Navigators take advantage of opportunities to check the direction of swells against the stars. When two swell systems are moving across each other, like the converging wakes of motorboats, they make peaks where they come together. Currents reveal themselves by the shape of the waves.