The calculations to work out longitude from time are very, very, basic. You just need your local time (LMT), and Greenwich Mean Time (GMT). Then you do these three steps:

- Local time - GMT = Time difference
- Time difference divided by 24 hours = Time factor
- Time factor multiplied by 360 degrees = Longitude

These steps can be reduced to a single equation:

## (LMT - GMT) * 15 = Longitude

It seems almost laughably simple, but the problem Harrison had to solve was that there simply weren't any clocks accurate enough to keep GMT on a sea voyage.

As a result, most of the establishment believed they would find a solution through astronomical observation, rather than a time, which no device available could measure.

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