You can get longitude without a clock, via lunar distances. It's just not an easy method, and didn't really become practical until the Nautical Almanac provided

You can get longitude without a clock, via lunar distances. It's just not an easy method, and didn't really become practical until the Nautical Almanac provided the requisite tables, not long before the first proper marine chronometers came into use. But people often forget how much mathematical training was needed to navigate in the age of sail.