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Centuries-Old Longitude Clock Runs Again
douglips writes "BBC News has published a story about John Harrison's H4 chronometer and how it has been wound up for the UK's National Science Week.
After 40 years of work [Harrison] proved in 1764 that a clock could be used to locate a ship's position at sea with extraordinary accuracy." Ah, the GPS system of its day. T. adds: This is the timekeeping device which Dava Sobel wrote about in Longitude .
The interesting thing about the Harrison clocks, is not only were they the GPS of their day, they were also the atomic clocks of their day.
The Harrison clocks, created in the 1700's, are still more accurate than your average digital watch today.
I was in London last November, and visited the Royal Observatory in Greenwich. I was familiar with the Harrison clocks and story, but I hadn't known they were kept there. So it was a pleasant surprise to find them there. If you're a geek and you happen to be in London, it's well worth your time to go take a look.
The first three clocks are these large (roughtly 1.5 ft in each dimension) contraptions with lots of visible moving parts, wooden gears, etc. Then you get to H4, and it's this elegant little package. The leap between the first three clocks and the fourth is enormous.
There's a fair amount of other neat stuff at Greenwich, too. They have a number of displays about the development of "time infrastructure". I remember reading one bit that talked about how, in 1852 (I believe), Greenwich began transmitting the time to the rest of England via telegraph. I couldn't help but be reminded of how clock signals are distributed around a CPU and other synchronous logic devices, and think that maybe humanity is somewhat more borg-like than we usually acknowledge.
Clocks in heavier gravitational fields tick at a slower rate.
Clocks in faster relative motion tick slower.
So:
A clock at the equator ticks slower than a clock at the north pole, because the relative velocity of objects at the equator is higher than those at the poles (the axis of spin) due to earth's rotation, but,
The equator clock will tick faster because it's located farther from the earth's center of mass (due to earth's spin, it bulges a bit in the middle) resulting in slightly lower gravity- and the effects don't always cancel each other out.
So then,
Relativity predicts that atomic clocks onboard GPS satellites will tick faster by about 50 microseconds per day (compared to ground-based clocks), due to the weaker gravitational field in orbit, but,
They also will tick slower by about 7.2 microseconds per day, due to the satellites' orbital velocity.
GPS's designers compensate for this by changing base time rate for the clocks onboard satellite.
Fun facts:
The cesium atomic clocks onboard GPS satellites are accurate to about one nanosecond, and light travels about one foot in one nanosecond. Hence, the best accuracy of GPS is about one foot.
GPS satellites have been used to experimentally verify that light moves at constant speed at all times/locations visited by earth.
And there are other confirmed predictions as well. One other I've heard is that GPS's radio signals experience frequency shift due to earth's gravitational field (photons want top accelerate but can't surpass C, so the acceleration energy increases their frequency) and this had to be compensated for as well.
Time be time.
Big Daddy, Johnny, Burp, Aunt Zelda, Scott, Slurp, Big Momma
Mechanical clocks and watches are still hand manufactured by a company in sweeden after 200 years. They are accurate up to 1/10 of a second per week and the spring mechanisms have gotten so advanced that they go for a month without rewinding them. This may not sound so impressive in a large clock, but consider that this is all done in a watch! The only downside is due to the lack of trained watchmakers and the fact that these are all handmade, each watch can run you several thousand dollars! But think of all the money you'd save on batteries.