Chapter 18: Time and Timekeeping
A four-second timing error displaces a sight by one nautical mile of longitude. Celestial navigation is fundamentally an exercise in precise time.
Kinds of time
Apparent solar time follows the real sun and is irregular; mean solar time follows a fictitious uniform sun. The equation of time is the difference, reaching about ±16 minutes. Universal Time (UT1) is mean solar time at Greenwich and is the argument of the Nautical Almanac; UTC is the atomic broadcast time kept within 0.9 s of UT1 by leap seconds. Sidereal time follows the stars, gaining about 3m 56s per day on solar time.
Zone time and date
Zone description is the whole number of hours to apply to zone time to get UT, named plus in west longitude and minus in east. Each zone spans 15° of longitude, centred on a standard meridian. Crossing the date line westbound advances the date by one day; eastbound repeats a day. Chronometer error and its daily rate are logged so that the watch time of a sight can be converted to UT with certainty.
- UT = ZT + ZD (west longitude, ZD positive)
- 4 seconds of time = 1′ of arc = 1 NM of longitude error at the equator
- Log chronometer error and rate daily against a radio time signal
Formula cards
Arc–time conversion
1h = 15°, 1m = 15′, 1s = 15″
Used to convert longitude to time and vice versa.
Zone time to UT
UT = ZT + ZD (ZD positive west, negative east)
Watch the date change when UT crosses midnight.
Key takeaways
- • The Nautical Almanac is tabulated against UT — convert every observation to UT first.
- • 1 hour = 15°, 4 minutes = 1°, 4 seconds = 1′ of arc.
- • Equation of time = apparent time − mean time, up to about 16 minutes.
Knowledge check
Immediate feedback with rationales. Answer each question to see the explanation.
Chapter 18 quiz
0 of 3 answered
1.Zone time is 0800 at ZD +5. UT is:
2.A four-second error in timing a sight produces a longitude error of about:
3.The equation of time is the difference between: