Will, a look of tried forbearance on his long face, suggested that he go up and keep lookout for Nathaniel. There were a great many vessels about.

“Aye-aye, second in command.” Eliot gave an exaggerated salute. “Maybe there’s a sense of humor above decks.”

“And now that we are free of distraction, we shall see how much Papa taught you,” Will said to Drew. “When you took the sight, the sun was ninety degrees over a certain point on the earth at that moment. That’s called its geographic position. Everything we use in navigation—sun, moon, stars, planets—has a geographic position, and we can look it up in here.” He motioned at the thick, formidable-looking Nautical Almanac. “Got that? You’d better, because what we do with the sextant is plot where we are in relation to that heavenly body’s position over the earth. And it’s real important, I would say very, no, critically important that you note the time of your sight as exact as you can. And why?”

“Because the sun and planets and stuff are all moving.”

“How fast?” asked Will. “If, say, you shot your sight, and then took four seconds to tie your shoe before you noted the time, how far would the sun have moved?”

“Pretty far, I’d reckon.”

“Give you a hint. The earth rotates on its axis at nine hundred miles per hour.”

Drew bowed his head, struggling to recall his father’s lectures of a year ago. The solution came to him when he stopped working for it, came to him, as solutions to mathematical problems often did, in a burst more like a revelation than an answer arrived at step by laborious step.

“One mile,” he replied, and Will’s approving nod warmed him inside—the same feeling he got when his teacher at Boston Latin congratulated him for solving equations, or when the minister at Sunday school applauded him for reciting the books of the Bible first in alphabetical order, Acts to Zephaniah, and then chronologically, Genesis to Revelations. He could not box or play football the way Nat did, nor row like Eliot, who was a star on the Andover crew, and so he craved and delighted in the rewards his mind won for him.

“A mile,” said Will. “That’s how far we’d be off if we dawdled four seconds before noting the time. Now, a mile won’t make much difference if you’re not going far, but sail far enough long enough on a mistake like that and you’ll end up in Africa, when you thought you were going to Ireland. Thus endeth that lesson.”