Every celestial fix is a time fix: four seconds of error are a mile of longitude. Three clocks run on an ocean passage, UT, ship's time and local mean time; always know which one you hold.
How do I convert longitude into time to get UT from local mean time?
At 15° to the hour: 1° is 4 minutes, 1′ is 4 seconds. Then UT is local mean time plus west longitude in time, or minus east longitude in time.
The Earth turns 360° in 24 hours, so longitude and time are one thing in two units. Local mean time (LMT) is mean time on your meridian; UT is LMT at Greenwich.
| Arc | Time |
|---|---|
| 15° | 1 hour |
| 1° | 4 minutes |
| 15′ | 1 minute |
| 1′ | 4 seconds |
UT = LMT + west longitude in time, or − east longitude in time.
The equation of time is the gap between the true Sun and the mean Sun, up to about 16 minutes. You never apply it yourself: the almanac's Mer Pass, 1144 LMT in early November, 1214 in February, is noon with it already in. Apply your longitude in time for the UT of local noon; timing the Sun at its highest and comparing with Mer Pass gives your longitude.
How do I convert ship's time to UT using the zone description?
Add the zone description: UT = zone time + ZD, where ZD is the longitude divided by 15 and rounded, positive west and negative east. Ship's time 1530 with ZD +4 is 1930 UT.
The world is cut into 24 zones 15° wide, centred on 0°, 15°, 30° and so on, with boundaries at 7°30′, 22°30′, 37°30′ … The zone description (ZD) is the longitude divided by 15 and rounded, positive west and negative east.
UT = zone time + ZD. Ship's time 1530, ZD +4: 1930 UT.
Keep the ship's clocks on the zone you are in: westbound retard them an hour at each boundary, eastbound advance them. A retarded night makes one watch five hours long, so split the hour among the night watches. Shore time is whatever the country keeps, summer time and half-hour zones included. Log sights, fixes and radio schedules in UT, which never jumps.
Do I gain or lose a day crossing the date line westbound?
You skip a day: crossing the date line westbound into east longitude, the date goes forward; eastbound you repeat one. Hours and minutes do not change, and UT and its date run on.
East of 180° the zone is ZD +12, west of it ZD −12: the same clock time, a day apart. Crossing westbound into east longitude you skip a day, the date goes forward; eastbound you repeat one. Hours and minutes do not change.
The deck watch on UT is not touched: UT and its date run on, and they are what the almanac wants. Convert first and open the Greenwich date, which may differ from the ship's: at 0745 on 15 March with ZD −12 the sight is at 1945 UT on 14 March. A passage time is the difference of the two UTs, not of the local dates.
How do I work out my deck watch error from its daily rate?
Error now = last error + rate × days, the rate being how the error changes each day. A watch 18 s fast and gaining 2 s a day is 28 s fast five days later.
A watch is never right; it is fast or slow by a known error, and the rate is how that error changes each day: error now = last error + rate × days. Heat alters the rate, so in the tropics use the rate from recent checks, not the one from Biscay.
Check against a radio time signal, WWV or the hourly pips, the hour being the start of the long pip; UTC is UT within a second, and the signal's travel time is nothing. Set the watch from a GPS only on its UTC display: raw GPS time runs 18 seconds ahead. Carry two or three watches and compare them daily: one that jumps shows against the others.
| Error | Effect |
|---|---|
| 4 s | 1′ of longitude |
| 1 minute | 15′ of longitude |
| Fast | GHA too big, fix too far west |
| Slow | fix too far east |
| Any | noon latitude unchanged |
A minute of longitude is a mile only on the equator: multiply by the cosine of the latitude. Time a sight with a stopwatch started at the instant and read against the watch after; a timekeeper notes the seconds first.
How do I get the Sun's GHA for a sight taken between whole hours?
Take the GHA for the whole hour of UT from the daily pages, by Greenwich date, then add the increment for the minutes and seconds from the increments table; correct Dec by d's share.
The daily pages give GHA and Dec for every whole hour of UT, by Greenwich date. The increments table adds the minutes and seconds, with columns for the Sun and planets at 15° an hour, Aries at 15°02.5′ and the Moon. The Dec changes by d an hour; for the minutes add or subtract d's share, the sign by inspection of the next hour.
The last second of an hour belongs to that hour's row, with 59 m 59 s of increment. A watch on ship's time is first turned into UT with its date: a sight at midnight can belong to tomorrow's page.
Are the almanac's sunrise, sunset and twilight times in UT or local time?
In LMT, against latitude: interpolate for yours, add west longitude in time or subtract east for UT, then take off the ZD for ship's time.
Sunrise, sunset, twilight and meridian passage are tabulated in LMT against latitude: interpolate for yours, add west longitude in time (subtract east) for UT, then take off the ZD for ship's time. Shortcut: ship's time is the LMT plus 4 minutes for every degree you are west of the zone's central meridian.
Star sights live between civil twilight, Sun 6° down, and nautical twilight, 12° down: horizon and stars together. In high-latitude summer the almanac prints //// where the Sun never gets that low: plan on sun lines. The Moon crosses the meridian about 50 minutes later each day: interpolate between the two days' values by longitude, west longitude towards the next day. A planet with Mer Pass near noon is lost in the Sun; one in the evening or early morning gives a twilight sight.
| Body | Meridian passage |
|---|---|
| Sun | Mer Pass on the daily page, LMT |
| Moon | Daily page, corrected for longitude |
| Planets | Daily page, LMT |
| Star | Mer Pass of Aries + (360° − SHA) in time |
Questions from this topic, as the app asks them. Open one to see the answer.
You are halfway across on the ARC, converting the almanac's LMT to UT by hand. What is your longitude in time?
A. 3h 30m
52.5° × 4 min = 210 min = 3h 30m (15° = 1 h, 1° = 4 min, 15′ = 1 min).
Your ship's clocks keep the Caribbean zone time. What do you write in the UT column of the log?
B. 1930 UT
West longitude, ZD +4: UT = zone time + ZD = 1530 + 4 h = 1930.
Bound west from Tonga to Fiji, you cross the 180th meridian at 2100 on Tuesday 9 June, ship's time. What do you write in the log a moment later?
A. 2100 Wednesday 10 June
Westbound across 180° the date goes forward a day (ZD +12 becomes −12); the clock time stays the same.
You take a sun sight on the passage from Bermuda to the Azores. What UT goes into the reduction?
C. 14 22 58
A fast watch shows too much: UT = 14 23 10 − 12 s = 14 22 58.
You work a sun sight by hand. How do you get the GHA for the moment of the sight?
B. The 14h GHA plus the increment for 23m 10s
The daily page gives GHA for each whole hour; the increments page adds the minutes and seconds (15° an hour for the Sun).
You plan the evening star sights off the Queensland coast. At what UT does the Sun set?
D. 0744 UT, 14 May
147°45′ = 9h 51m; east longitude: UT = LMT − 9h 51m = 1735 − 0951 = 0744 UT, the same date.
All 100 questions of this topic are in the app: practice by topic with the reason after every answer, your mistakes brought back, and a timed mock exam. Free, no sign-up, in the browser or in Telegram.