Marine Glossary

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rocking the Sextant

A motion of the sextant that must be done when taking a sight to insure that the proper vertical height of the object is measured. "Rocking" means rotating the sextant, without moving the head, about an imaginary line from your eye to the image of the object on the horizon. For actual sights, you need rock very little, some 20° or so, to either side, but it is good to practice rocking even farther to begin with as this teaches you how to keep the object in view on a moving boat.

As you rock the sextant you must willfully keep it pointed toward the object, since the tendency is to rotate as you rock (yawl as you roll). As you rock, the image will appear to move along an arc. You want the object to just touch the horizon at the lowest point of the arc. Achieve this by adjusting the micrometer as you rock the sextant. See Sextant and Sextant Sight.