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Coastal navigation

Time at sea – from UTC to apparent local time

On board, several clocks run side by side: UTC for radio, weather and celestial navigation, ship's time for the watch system, the legal time of the next port – plus the times that follow from your own longitude. NauticTools2 works them all out from a single UTC moment and your longitude.

• UTC
• Zone time
• Longitude in time
• Local mean time
• Apparent time
• Equation of time

Which time systems are there?

• UTC (Coordinated Universal Time) – the common reference, formerly called GMT. The nautical almanac, radio traffic and weather reports all use UTC.
• Zone time – the earth is divided into 24 zones of 15° of longitude, one hour apart. Zone time is the standard time of the zone you are in.
• Legal time – the time a country sets officially, including daylight saving time. It is a political decision and cannot be derived from position.
• Ship's time – the time life on board runs by. The skipper sets it and, on long passages, changes it step by step.
• Longitude in time – your longitude expressed in hours and minutes.
• Local mean time (LMT) – the time of your own meridian, based on an imaginary, evenly moving “mean sun”.
• Local apparent time (LAT) – the time a sundial shows at your position. At 12:00 LAT the sun is exactly due south (or due north).
• Equation of time – the difference between apparent and mean time.

When do you need it?

• Celestial navigation: The positions of celestial bodies in the nautical almanac are tabulated against UTC. If you note the time of a sight in ship's time, convert it to UTC first.
• Planning a noon sight: The sun culminates at 12:00 apparent local time. With the equation of time and longitude in time you get the UTC and ship's time of the noon sight.
• Long passages: Sailing east or west you cross the zones. Ship's time is then changed an hour at a time, usually at night, so that daylight and watches stay in step.
• Radio, weather and port: Weather reports and navigational warnings come in UTC, while harbour offices and locks work to legal time.

Formulas and working

East longitude counts positive, west longitude negative. The individual steps, as the working in NauticTools2 shows them:

1. Zone number

   Longitude divided by 15°, rounded to the nearest whole number: zone = λ / 15°

2. Zone time

   ZT = UTC + zone number (in hours)

3. Longitude in time

   15° equal one hour, so 1° = 4 min and 1′ = 4 s.

4. Local mean time

   LMT = UTC + longitude in time

5. Equation of time

   The difference between the true and the mean sun, expressed in time. NauticTools2 calculates it for the exact moment from the position of the sun.

6. Local apparent time

   LAT = LMT + equation of time

7. Legal time and ship's time

   UTC + the offset you enter, for example +1 h for British Summer Time.

Zone description: In British and US practice the zone is often given as the zone description (ZD), the correction from zone time to UTC – with the opposite sign. Zone −2 in NauticTools2 corresponds to ZD +2: UTC = ZT + 2 h. The old rhyme still helps: “Longitude west, Greenwich time best; longitude east, Greenwich time least.”

Worked example

Things to watch out for

• Signs: East of Greenwich local time is later than UTC, west of it earlier.
• Date: At large longitudes or close to midnight, the date changes along with the time. That is why NauticTools2 shows the date with every time.
• Sign of the equation of time: Some textbooks define it the other way round (mean minus apparent time). Here: apparent time = mean time + equation of time.
• Don't guess legal time: Many countries differ from “their” zone – Spain and France, for example – and daylight saving time comes on top. Look the offset up and enter it.

How it works in NauticTools2

1. Enter UTC

   Date and time in UTC. Short forms such as 100926 for the date or 1432 for the time are completed automatically.

2. Enter longitude

   Degrees, minutes and east/west. If a GPS position is available, the longitude is already filled in.

3. Enter offsets (optional)

   Legal time and ship's time as an offset from UTC, for example +2:00 or -1:30. Leave them empty and NauticTools2 works out only the times that follow from longitude.

4. Calculate

   All time systems appear in one list, with the working below. Inputs can be saved, loaded and printed as a calculation record.

Frequently asked questions

How do you convert longitude into time?

15° of longitude equal one hour, so 1° = 4 minutes and 1′ = 4 seconds. 023°15′ W is 92 minutes plus 60 seconds, that is 1 h 33 min.

What is the difference between zone time and legal time?

Zone time follows from longitude alone. Legal time is set by each country, often different from the zone and with daylight saving time. At sea you keep zone time, in port legal time.

Why does a sundial differ from local mean time?

The earth moves on an elliptical orbit and its axis is tilted, so the true sun crosses the sky unevenly. The equation of time makes up for this. In February a sundial is about 14 minutes slow, in early November about 16 minutes fast.

Related tools

• Meridian Passage – when the sun crosses your meridian
• Ephemeris Calculation – positions of celestial bodies for a UTC moment
• Latitude by Meridian Altitude – latitude from the sun's altitude at noon