Marlon Brando screamed: “We will never leave here without it!” — but the ship sank with the precious instrument still onboard.

Haunted by the image of Brando leaping into the flames to try to rescue the device, young David asked what a sextant was — and why it mattered so much.

Later, as a 19-year-old crossing the North Atlantic in a 35ft sloop, he would find out for himself how the small device helped sailors track their way across vast oceans using the positions of the sun, moon and stars.