Part I · Foundations · Page 3 of 4
Why the seconds matter
Because 4 seconds of time is 1′ of arc, a watch that is 4 seconds wrong puts your position up to a mile east or west of where you are. Navigators keep a careful record of how fast or slow their watch runs (its watch error) and correct for it before converting to UT.
Predict Your watch is 8 seconds fast and you forget to correct it. Where does your fix go?
- Watch error, not corrected
- 20 s fast
- Longitude error
- 5.0′ west
- At latitude 40°, that is
- 3.8 nm
Every 4 seconds of time is 1′ of longitude. Latitude from a noon sight or Polaris does not depend on the time, so only the east–west position moves. Near the poles a minute of longitude is short, so the same error matters less.
A watch also gains or loses at a steady rate, so the error grows day by day. Check it against a time signal or GPS, write down the error and the date, and work out today's error from the rate:
On March 1 the watch was 12 s fast. It gains 1.5 s a day. On March 21, 20 days later, it has gained 30 s more: it is 42 s fast.
So a reading of 14h22m50s by the watch is 14h22m08s zone time.