Here is a complete list of the required parts; For the Electrics; Box/Case Small servo Arduino Duemilanove HMC6352 Compass Module http://www.sparkfun.com/products/7915 Mini breadboard Female-female hookup wire Male Headers ...
This was trial and error. I went to a local clock repair shop and got cogs that geared up from 1 to around 4. This gave me a 90 degree turn on the servo equal to roughly 360 turn on the final cog.
To test the positioning of the cogs before I started putting them into the final box I put nails through them and tried them out on some scrap wood.
To begin with I drilled a hole in the lid of the box big enough to fit the shaft of the servo and screwed the servo to the inside of the case. On the outside I then screwed and glued the main drive cog directly into/onto the shaft of the servo.
All other cog are free to rotate around simple screws.
The final cog has an hour hand from a clock glued to the top of it. This allows the cog and hand to move together freely around the final screw.