The Age of Exploration: Chapter 5, Exercise 3 — Possible Solution
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CALCULATION
The Earth rotates 360 degrees in 24 hours, so each hour of time difference is
15 degrees of longitude. A 2-hour difference is 2 x 15 = 30 degrees.

WHY TIMEKEEPING WAS THE KEY PROBLEM
To find longitude you must compare local time with the time at a known place,
so you need a clock that keeps accurate time at sea. Pendulum clocks were
impractical on ships, which is why Harrison's chronometer mattered.

CAUTION
The sources do not say which way (east or west) the difference points; that
depends on which place is ahead in time.