The sextant is the treasured navigational device of generations of seamen and airmen. It enables position to be determined with good accuracy for ocean navigation, and it was the combination of sextant and chronometer that allowed safe navigation of the oceans for generations of seafarers.

The 13th edition of this bestselling book is proof of the success of Mary Blewitt's concise and clear style in explaining a particularly difficult skill, and it has been the bible for many generations of ocean navigators.