Melville explains that sperm whales swim in veins -- predictable pathways from one feeding-ground to the next. Therefore, it is possible to plan a course for a whaling voyage that will maximize the probability of encountering the prey. But Ahab, in addition to this, also knows that Moby Dick, for several consecutive years, has been encountered in the Pacific Ocean at the equator during the so-called "Season-on-the-Line" ["the line" being the equator]. This season, according to Melville/Ishmael, runs from approximately December to June: "as the sun, in its annual round, loiters for a predictable interval in any one sign of the Zodiac". [With regard to the reference to signs of the Zodiac, when the sun seems to stop getting lower in the sky around December 22, it is at Winter Solstice and would appear to be directly overhead to a ship on the Tropic of Capricorn (23.5 degrees south latitude). This would be the beginning of the Season-on-the-Line, which would extend to June 21 when the sun is overhead at the Tropic of Cancer (23.5 degrees north latitude)].
Therefore, Ahab would have the best chance of spotting Moby Dick in the equatorial waters of the Pacific sometime between December and June. However, the Pequod sailed from Nantucket at Christmastime in December -- too late to make full use of those six months of that year for finding the white whale. [Note that Melville suggests that Ahab decided the season in which to begin the voyage, but owners, not captains, are more likely to decide this.] So Ahab will bide his time for a year, plotting a course from Nantucket around the African Cape (rather than the South American Cape), then through the Indian Ocean to the Sea of Japan, whaling all the while, hoping to see Moby Dick by chance -- and finally sailing south to arrive at the equator in December of the following year where the big white whale is sure to be waiting. All this plotting, scheming, and conniving caused Ahab insufferable anguish. So tormented was his sleep that "a wild cry would be heard through the ship; and with glaring eyes Ahab would burst from his room, as though escaping from a bed that was on fire". Melville / Ishmael then gives us a next-to-incomprehensible psychological / metaphysical diagnosis of Ahab's condition:
It was not actually Ahab himself, that would burst from his hammock in terror. Instead of Ahab, it was "the eternal, living principle or SOUL in him; and in sleep, being for the time dissociated from the characterizing mind -- his SOUL sought escape from the scorching, frantic thing of which, for the time, it was no longer integral."
This is getting very deep, but it gets deeper. If it wasn't really Ahab that jumped out of bed to run on deck, what was it? -- and what on earth was left in the hammock? Well, it was Ahab's purpose that assumed a life of its own and stayed in bed, while "what seemed Ahab rushed from his room, was for the time being a vacated thing, a formless somnambulistic being, a blankness in itself". To top this off, Ahab is declared a Prometheus [thief of the fire of the gods], doomed to the eternal torment of being eaten by the vulture of revenge he has created.