Decide
Your deck watch is 14 s slow and reads 10-14-12 at a sight. What is the UT?
The lesson in plain words
- Why time matters
- The Sun's GHA changes by 15′ every minute of time, so 4 seconds of time error gives 1′ of longitude error: up to 1 M at the equator.
- A noon sight for latitude hardly depends on time, but every other sight needs the UT of the observation to the second.
- UT and UTC
- The almanac is tabulated in UT, Universal Time.
- Radio time signals and GNSS receivers give UTC, kept within 0.9 s of UT1, the rotation-based scale.
- Treat UTC as UT; the small difference, DUT1, matters only in very precise work.
- Zone time and the zone description are in Navigation: Polaris and time. Log all sights in UT.
- Chronometer and deck watch
- A chronometer is a precise clock kept below and never reset.
- A deck watch, or a good quartz watch, is compared with it or with GNSS and taken on deck for sights.
- Its error is how much it is fast or slow on UT: UT = watch − fast error, or + slow error.
- Write it down at every check.
- Rate
- A watch's rate is how much it gains or loses each day.
- Keep a time log of date, error and daily rate.
- For calculations, make fast errors and gaining rates positive; slow errors and losing rates negative.
- Predicted signed error = last signed error + signed daily rate × days.
- Positive means fast; negative means slow.
- A steady, known rate matters more than a small one.
- Taking the time
- Check the watch daily against GNSS or a radio time signal; NP282 lists time signals.
- A helper can read the watch when you call 'stop'.
- Alone, start a stopwatch at the sight.
- Later, read the watch and stop the stopwatch together.
- Subtract the elapsed time, then apply the watch error.
- Check AM or PM on a 12-hour dial and confirm the UT date.
Key data
- Time and longitude
- 4 s of time = 1′ of longitude; 1 min = 15′
- UTC and UT1
- Within 0.9 s (DUT1): treat UTC as UT
- Watch error
- UT = watch − fast error, or + slow error
- Predicted error
- Signed error + signed rate × days; fast/gain +, slow/loss −
- Time signals
- Listed in ADMIRALTY List of Radio Signals NP282
- Spanish terms
- Cronómetro, estado absoluto, movimiento diario
Key points
- 4 s of time = 1′ of longitude: time sights to the second.
- The almanac is in UT; treat UTC from GNSS or radio as UT.
- UT = watch − fast error, or + slow error.
- Keep a time log: error and rate; a steady rate is what you need.
- Check the hour and the date: a 12-hour dial can be 12 hours out.
Check what you have read
An 8-second error in the time of a Sun sight gives a longitude error of about:
True or false?
Decide each statement before you see the answer.
UT = watch − fast error, or + slow error.
Check the hour and the date: a 12-hour dial can be 12 hours out.
The deck watch keeps zone time, so the sight can be worked in zone time.
A watch that is fast must have its error added.
Practice case & exam keys
Situation
On 20 November 2026 your deck watch was 4 s slow on GNSS UTC, losing 1 s a day. On 26 November the GNSS fails before a morning sight. The watch reads 10-14-17 at the sight. What UT do you use?
Show the answer
Six days at 1 s a day losing adds 6 s: the watch is now 4 + 6 = 10 s slow. Add a slow error: UT = 10h 14m 17s + 10 s = 10h 14m 27s, on 26 November. An error of a few seconds in that estimate costs about 1′ of longitude per 4 s.
Exam keys
- 4 s of time = 1′ of longitude; time every sight to the second.
- The almanac uses UT; UTC within 0.9 s is treated as UT.
- Watch error: subtract fast, add slow. Rate predicts the error between checks.
- Check the hour and the date before entering the almanac.