Despite a common misperception, Marine Chronometers are not the perfect timekeepers.  What makes them so special is that they ARE nearly perfect rate keepers.

What this means is that while a chronometer is most unlikely to agree with standard signals sent from the US Naval Observatory, it has the ability to maintain a very consistent rate.  In other words, its measurement (calibration) error is: known, consistent, predictable, and (ideally) small.

As an illustration, many common clocks show the time to within a few seconds over a week.  Yet, within the week, the clock may be off by as much as 15 minutes.  There are a number of reasons for this, but it boils down to quality of construction and attention to detail.

A marine chronometer is constructed and used in such a way that its rate does not swing so wildly over time.  If it is .5 seconds per day slow one day, it can be counted on to be .5 seconds slow per day a year later.  Equally important is that that .5 second loss will be evenly divided across the the full 24 hours.  The .5 second error won't all occur just after it is wound, for example.

This attention to detail is not surprising when you consider that ships (or railroads) depended on precise timing for operations.  For navigators who are at sea for two to three years before being able to check their chronometer against a known time signal, being able to predict the error of the chronometer was extremely important.  He would multiply the known error by the the number of days at sea, and adjust the dial reading accordingly to arrive at the time of day in Greenwich, England. 

Marine chronometers are hung in gimbals for the express purpose of keeping them in a "dial up" position.  This is not for the convenience of the navigator.  When the chronometer is dial up (or down), it is in a position that eliminates errors due to the balance being out of balance (poise).   By eliminating the need to worry about poising errors, all of the maker's efforts on the balance assembly could be directed to ensuring the chronometer maintained a consistent rate across the wild extremes of temperature encountered by a ship sailing from Maine to the Antarctic.

You see, the adjustments made to correct the poise of the balance are the same adjustments made to ensure stability across temperature.  These adjustments involve moving the position of weights around the rim of the balance wheel.  The adjuster has to decide which is the more important goal: Temperature compensation (for marine chronometers hung in gimbals) or Position Adjustment (for railroad watches kept at a constant temperature in a pocket).

Since Railroad Watches are worn in the pocket, and kept at constant temperature, the adjuster has more freedom to worry about position adjusting.  Which is a good thing since the railroad worker is not likely to be sitting on one position.

Typical results I achieve:

• Marine Chronometers
  • Hamilton M21              Less than 1 second per day
  • Non-Hamilton               Less than 2 seconds per day
• Precision Watches
  • Hamilton M22              Less than 1 second per day
  • Other Deck Watches   Less than 3 seconds per day
  • Railroad Watches        Less than 30 seconds per week