A couple of weeks ago, while our friends David and Jen were visiting from Boston, we all went to Greenwich to visit the Royal Observatory.

Greenwich is probably best known as the baseline for both the Prime Meridian and the Greenwich Mean Time: 0º longitude and 0:00 hours.

And, of course, the canonical things to do when you’re there are to stand on the zero line…

Susan half in one hemisphere and half in the other.

… and to set your watch by GMT:

The Royal Observatory clock. Note the prime meridian line running vertically through the clock.

Sadly, these days the public clock display is set from GPS time, rather than an on-site master atomic clock.  Oh well.

While it may not be entirely obvious, the fact that 0º and 0 hours are co-located is not at all a coincidence, nor is it just the whims of the British Empire in action.  The two are actually quite fundamentally linked, at the level of physics and math.  The small observatory museum had a fascinating discussion of the science and technology of location and horology.

The short-short story is that knowing your longitude can be reduced to knowing what time it is.  Or, more precisely, knowing what time it is where you are relative to what time it is where you started.  For example, if you can figure out that when it’s exactly noon to you, it’s 11:00 AM at your starting point, then you know that you’ve moved 1/24′th of the Earth’s circumference east.  (More precisely, you’ve moved 15 degrees east, which is 1/24th of the Earth’s circumference, or roughly 1670 km, only at the equator.  Close enough for this discussion, anyway.)  So knowing time gives you distance.  Therefore, it makes perfect sense to put your 0 time base on top of your 0 distance base.

Now, where you choose to put 0 longitude is a matter of convenience or, in practice, the might of the British Empire.  But having done that, your time base is naturally set.

This is a familiar story to anybody who has read a bit of the history of technology or exploration.  What I found really interesting was the museum’s history of horology, focusing on the Harrison chronometers of the Eighteenth Century.  These were the first timepieces with sufficient precision and robustness to be used in practice for navigation.  He built a series of chronometers in response to a contest to devise such a device, and the Royal Observatory Museum holds originals or replicas of the first four models, most in working condition.  (The fourth, “H4″, ultimately won the contest.)  They are fascinatingly intricate machines:

Harrison's first contest chronometer. State of the art for the day, but not quite good enough.

(Sadly, we weren’t allowed pictures in the museum itself, so this is a stock image grabbed from the ‘net.)

What really startled me, though, was the evolution of the chronometers.  The H1, above, is about 3 feet tall, made of delicately machined brass.  I have trouble guessing how heavy it is, but I’m sure that it would take a couple of people to move it around.  H2 and H3 were similarly intricate, delicate, and bulky.  But then, just a few years later, Harrison released the H4:

Harrison's H4, ultimate winner of the contest.

Looks like a pocket watch, doesn’t it?  In fact, it is the great grandfather of all pocket watches — within a few years, it had been duplicated, miniaturized and was being mass produced for individual use.  The H4 itself is too large to be a convenient pocket watch (you probably can’t tell without a scale, but it’s maybe 4 or 5 inches across), but compared to its predecessors it’s a model of compactness and elegance.  It was incredibly startling to see three massive beasts of chronometers lined up, followed by something that looked nearly petite in comparison.  It is both a reminder that recent memory is not the only era in human history when technology has advanced blazingly quickly and a vivid demonstration that getting the core design ideas right makes a huge difference in the final design.