Definition
Rotating the instrument slightly about the line of sight makes the body's image swing through an arc; the lowest point of that arc is the moment the sextant is truly vertical. Reading at any other tilt gives an altitude that is too large. The observer adjusts the micrometer until the limb just grazes the horizon at the bottom of the swing, then calls the time to the second for the timekeeper to record.
Want to apply this on the water?
Ask Saily out loud for an example, a scenario or a simpler explanation.
Talk to Saily