"Greenwich Observatory is famous for time, of course, and it's 1: CHOOSE formost famous infamous far most for having the Prime Meridian but the reason it was founded in 1675 was to improve navigation, not ostensibly astronomy or horology but navigation. And the problem that they faced was not being able to determine their longitude when out of sight of land. Thousands of people were dying year by year and millions of tons of 2: CHOOSE variable venerable valuable virtual cargo were being lost because they couldn't navigate accurately because they didn't know their longitude.

Now longitude is how far around the world you've come from home and that is the same thing as time difference between where you are and back home. The simple answer to longitude is to know what time it is back home at the same time as you know your own 3: CHOOSE low cal loco local loaded time. Finding your own local time is dead easy, you just look at the position of the sun in the sky. If at that same instant you know what time it is back home you've solved the longitude problem. But how on earth do you know time back home? The 4: CHOOSE obvious oblivious of pious above us way is to take a clock with you which you set to home time before you leave but in the 1670's no one thought a clock could be made that could suffer the rocking of the ship and the wide variations in temperature.

So there was another method which was using the sky as a clock, the motion of the moon like a kind of hand against the stars of the 5: CHOOSE day dye dale dial of the clock. The Observatory was founded to make tables of the motions of the moon which would give the mariners out in the middle of the ocean Greenwich time by the positions of the stars."

What was John Harrison's contribution?
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How different was this from what had come before?
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Is Harrison's work with chronometers still relevant today?
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"Yes, Harrison's is the grand-daddy of the 9: CHOOSE modest modem moderate modern marine chronometer and the precision watch and you can see direct evolution from his fundamental ideas. His series of prototypes are all still here at the Museum and we preserve them with the first three of them actually running in the galleries.

The first three are not as accurate as they were because of course they're 250 years old and more. Obviously we don't like to make adjustments and permanent alterations to them because they're very important relics now. We regard them rather as relics than as time-keepers. But they're still 10: CHOOSE petty pity pretty pre-tea good considering their age."