Marine Glossary

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Watch Error

The difference between watch time (WT) and the standard time or zone time the watch is set to. The technical meaning is the same as the common meaning: how much any watch is fast or slow.

If your watch is set to Pacific Daylight Time (PDT), and it reads 14h 12m 20s, when you hear the proper time announced from a reliable source to be 14h 12m exactly, then your watch error is 20 seconds fast. If you hear Universal Time (UTC=GMT) announced as 21h 12m exactly, you learn the same thing, since PDT is 7 hours behind UTC. Since every watch has some rate of gaining or losing time, watch error increases as a voyage progresses. The size of the watch error is not important as long as your know what it is. Watch error can be checked with the WWV time broadcasts, or from computer or cell phone times as long as they are connected to a network. Accurate time is also available from any GPS device, or online from time.gov

To hear time-tic broadcasts on the telephone, dial (303) 499-7111 for WWV (Colorado), and (808) 335-4363 for WWVH (Hawaii). Callers are disconnected after 3 minutes. These are not toll-free numbers. See WWV, Watch Rate, Chronometer Log, and Universal Time.

As an example of the application of these corrections: I have a watch error (WE) of 3s fast, the zone description of my watch (ZD) is +7, and my watch time (WT) reads 13:34:08, so what is UTC? Answer: 13:34:08 - 3s + 7h = 20:34:05. If the answer is greater than 24 hr, then subtract 24h and increase the date by 1.

Generally watch rates are labeled + or -. For example, my watch has a rate of +0.350 s/10days. If it were losing time at this rate, the rate would be -0.350 s/10days. Then in practice we need to know when the watch was set to be correct, which we try to do on some day easy to remember, such as July 4, or our birthday, or so on. Say it is July 4, 2019. To use this information in the future, we need to know how many days have passed since it was set.

Suppose we need to figure WE on Oct 10, 2019, when the rate was -0.350s/10days. This means the watch is losing time, so when we read the error on some later date the WE will be slow. Now we can count out the days, but it is easiest to have a day of the year (DOY) table for this. There is one in every nautical almanac. Then we see that July 4 on 2019 was day 185 and that Oct 10 is day 283, which is 98 days, and 98 x 0.350/10 = 3.4s, which we can round to 3 sec, so the WE = 3s Slow. No matter how hard we try, we will never be able to keep track of this to the tenth of a second when underway.

Now if we want to make a generalized equation, we could write:

UTC = WT + WE + ZD, where both the WE correction and the ZD correction could be + or - . If WT is fast, then WE is -; if WT is slow, then WE is +. ie WE = 3s slow, ZD = +7, and WT = 13:34:05, we would have

UTC = 13h 34m 05s +3s + 7h = 20h 34m 8s.

The practical key is do not record WE with a ą sign, but rather use Fast or Slow. Then we will make the correction in the logical direction. Using ą signs can lead to confusion.

See: sextant sight, standard time, time zone, universal time, watch error, watch rate, watch time, zone description, and zone time.

See also: Time Keeping in Navigation and Weather.

Abbreviation: WE