The book that turned the Moon into a clock

The first Nautical Almanac reached the printer's customers in January 1767, and it let a sailor find his longitude by measuring how far the Moon had moved.

It tabulated the Moon's angular distance from bright stars every three hours of Greenwich time. Measure that angle at sea, look up when Greenwich saw the same one, and the difference is your longitude — the Moon used as the hand of a clock nobody can break. Every figure was worked twice by two people and checked by a third, who was called the comparer.