Monday, August 23, 2010

Gakken Kit #28, Edo-era Clock Explained

Ok, having received one question about how Gakken's kit #28, the Edo-era clock, works, I figured it was time to try to answer that for myself. Then, the more I tried to frame the answer simply, the more I realized that this was going to take over the 20 words normally associated with a blog reply. Hence, the dedicated, stand-alone post.

The background:
According to multiple sources, the primary one being the Japanese Clock entry on wikipedia, early time keeping in Japan consisted of burning columns of incense, and marking off the time in units of 6. Numbering for time started at "9" at the top of the new incense column and burned down to "4" at the bottom (numbers 3, 2 and 1 were reserved for Buddhist calls to prayer). What was important at that time was the amount of daylight available for farming. So, a "day" started with sunrise (when the stars were no longer visible in the sky) and ended with sunset (when the first stars became visible again). This led to two different counting systems - the 6 time periods during "day" and the 6 during "night".

Because the length of the day changes with the seasons (the longest "days" being in the Summer and the shortest in the winter), time keeping had to constantly be adjusted every few days to keep up with the actual occurrences of sunrise and sunset. As an example, on August 21st, 2010, sunrise in Tokyo occurred at 5:04 AM. Sunset at 6:24 PM. This gives a "day" of 800 minutes, and a "daytime hour" (800/6) of 133.3 minutes. Plus, a "night" of 640 minutes, with a "nighttime hour" that is 106.6 minutes long. Just 1 week later, on the 27th, sunrise was at 5:09 AM and sunset at 6:16 PM, with the "hours" changing to 131.2 and 108.8 minutes respectively.

"6" was at sunrise and then again at sunset, so a farmer would get up at "6" in the morning, work from "5" to "4" to "9" (when a new column of incense was started) and then back down to "6". It would now be sunset and the end of the "day". "Night" would run from "6" down to "4", then jump back up to "9" again with new incense and work back down. In addition, the hours were given the names of the animals from the Chinese lunar calendar, with "6" being the "hour of the rabbit", "5" being the "hour of the Dragon", etc. (the others being snake, horse, ram and monkey).

Then, in the 1500's, Dutch and Portuguese traders arrived in Japan, bringing with them the technology for western-style clocks. In 1641, Japan entered its isolationist phase, and Japanese clock makers were left on their own to come up with their own timepiece mechanisms. This led to the mechanization of the earlier incense-based time keeping system.

The What:
The Edo-era kit from Gakken has a clock face divided up into 12 equally-spaced units, running from 6 at our "9:00" position on the left, with 9 at "12:00", and 6 again at "3:00". The bottom half of the clock is the same, with 9 at the "6:00" position. The top half represents daytime, and the bottom half is night. The clock only has the "hour" hand, making timekeeping kind of approximate at best. In order to convert Edo time to modern western time, we need to know when sunrise and sunset are. If the clock is at roughly "day" 4.2 on August 21, then that would be 5:04 AM + (6 - 4 + 0.2) * 133.3 = 10:40 AM.

Again, the clock would need to be readjusted every 2 weeks or so to match up with sunrise and sunset as they changed with the seasons (this would usually be done after the clock ran down and needed rewinding).

The How:
Effectively, we need two clocks. One that is calibrated to advance at one rate from sunrise to sunset, and the second calibrated to advance from sunset to sunrise. For the design we're interested in right now, we want to use a horizontally-rotating pendulum, meaning that our clock is going to have two horizontal pendulums. (Stand-mounted clocks were weight-driven; smaller designs for sitting on a shelf or table were wind-up spring-driven).

The trick then is to switch between the two pendulums. This is accomplished by adding a gear that advances whenever the hour hand hits "6". This gear causes a cam rod to rotate 90 degrees. There are two cams on this rod. One cam raises its pendulum bar up out of the way of the escapement, while the second cam lowers its pendulum down into place.

(Sideview of the clock showing the two horizontal pendulums. Underneath them you can see the cylindrical sawtooth gear. The cam ensures that only one pendulum engages the sawtooth gear at a time.)

(Closeup of the sawtooth gear with the "daytime" pendulum engaged.)

Only one pendulum interfaces with the escapement at any given time. You then adjust the time by turning the threaded weights on the two pendulums to make them rotate faster (weights closer to the center of rotation) or slower (farther from the center). The original Edo-era clocks used standardized weights at fixed locations on the pendulum arms.

The final enhancement is to add a gear mechanism to ring a bell "on the hour", which on the Gakken kit is implemented by connecting the leaf switch to the Japanino (the Japanino then drives a servo which swings a brass rod at the bell).

I hope this is clear enough. For a better understanding of what the actual components look like, check out the Gakken site's assembly instructions.


Nick said...

Hey there Chris,

Once again, your entry was a big help to understanding the clock.

Thank you!

TSOTE said...

No problem. Glad to be of help.

I think one of the first mods for this kit should be to use acrylic paints to dress up the face of the clock and make the characters stand out better (either that, or cut-out contact paper). That way you can read it easier.