It should be obvious from the foregoing information in this Chapter that for the purposes of Astro-navigation an accurate timekeeping instrument is an essential part of the equipment of any well-found vessel. By ‘accurate’ it is meant the instrument should have a rate (i.e., the amount the instrument gains or loses each day) preferably not exceeding 5 seconds per day but a rate slightly higher than this is acceptable providing it is constant.

The traditional timekeeping instrument used on large ocean-going vessels of the Royal and Merchant Navies is the chronometer. This is a timepiece made as nearly perfect as human ingenuity and mechanical skill is capable of accomplishing. Mounted in a very substantial case (fig 8-1) slung on gimbals to maintain a horizontal position and the case placed in a well-padded box. 

The most modern ships’ chronometers are marine versions of the Quartz Integrated Circuit Clock, which are accurate to plus or minus 60 seconds per year at a normal constant operating temperature (which is not normally the case in vessels operating worldwide). They are powered by a single 1.5-volt battery, which can ensure independent operation for up to three years. The standard types of ship’s chronometer (whether traditional or quartz integrated circuit) seldom gives good results in a small vessel owing to the sometimes-violent motion in heavy seas. The so-called ‘Deck Watch’ or chronometer watch used by the Royal Navy is like a large pocket watch mounted on gimbals inside a felt-lined containing box with a glass window, and this will usually give better results.

Small quartz crystal chronometers specially designed for yachts, motor cruisers, and inshore fishing vessels are now becoming available, but the advice of a reputable marine instrument dealer is recommended. Whichever main timepiece is selected, it should be kept below deck as near to the centre of the vessel as possible where it will be subject to the least motion.

The main timepiece should not be moved from its designed stowage when at sea. A stopwatch with the usual large seconds hand and small minute hand is convenient for taking sights. Its use for this purpose will be described later in this study.

Regardless of the Zone or ‘clock’ time, a vessel’s main timekeeping instrument is kept continuously on G.M.T. on all voyages so that navigators can calculate their position from observations of celestial bodies. G.M.T. being the ‘standard’ time with which entries into the N.A. are made. 

When a chronometer or chronometer-watch is supplied it is customary for the nautical instrument supplier to furnish the navigator with a Chronometer Rate Certificate on which is written the error and rate of the instrument. During an ocean voyage, it is important that a careful check is made, and a record kept, of the chronometer rate. If this becomes erratic or unduly large, chronometer times used for Astro-navigational purposes should be treated with caution. A chronometer is regarded as being unfit for navigational purposes when its daily rate is erratic or when its daily rate exceeds six seconds per day.

Nowadays, radio time signals are available at any time of the day in all parts of the world, and afford an invaluable means of ascertaining the error and checking the rate of chronometers and watches. Marine versions of the quartz clock are now available as chronometers with rates as accurate as 5 seconds per month, and perhaps it will not be long before ships carry a small version of the atomic clock, whose accuracy is in the neighbourhood of a fraction of a second in several years, but for small vessels the expense of such a luxury would probably be prohibitive.

35. Radio Time Signals

The chronometer, chronometer-watch or deck watch used for timing Astronomical observations should be checked frequently (at least once per day) by Radio Time Signals. Full particulars of Radio Time Signals are given in the Admiralty List of Radio Signals, Vol. 5.

There is an absence of uniformity in the systems used for transmitting radio time signals, but in 1955, the International Astronomical Union recommended the use of the method by which time signals controlled by the Royal Greenwich Observatory are transmitted. This method has become known as the English System and it is envisaged that in time it will replace all other methods.

In the English system, time signals are radiated for five minutes preceding each hour of G.M.T. and consist of a series of 0.1-second dots at each second, each whole minute being signified by a dash of 0.4 second to facilitate identification. The commencement of each dot or dash is the timing reference point. Other systems of Radio Time Signals are described in ALRS Vol. 5. Many radio signals are operated automatically be mechanism connected to the Standard Clock of an Observatory. The accuracy of such signals is usually correct to within 0.05 second. At some radio stations, the time signals are sent by hand. The operator obtains the time from the Standard Clock at the radio station, which is checked either by Astronomical observations or by other radio time signals. These signals are usually correct to 0.25 second.

Of particular interest is the time signal transmitted by the BBC. This consists of the automatic transmission by the Standard Clock at the Greenwich Observatory of six dots (or pips) representing successive seconds before the whole hour of G.M.T., the final; dot being slightly longer (0.4 second) than the preceding five and representing the timing reference point. This signal is usually accurate to within 0.1 second. The Long and Medium Wave transmissions of the BBC can provide radio time signals for vessels in most northern European waters, which broadcast throughout the world on a large number of short-wave frequencies, will be found to be a most useful source of radio time signals.

36. The Rating of Chronometers and Watches

The difference between two errors of a chronometer divided by the time elapsed between finding them is called the rate. Thus, the amount by which a chronometer gains or looses during 24 hours of Mean Time is called the chronometer’s daily-rate.

 The rate over longer periods of time is called the accumulated rate, so that in cases where it has not been possible to check the error of the chronometer each day by radio time signals, the daily rate can be found from the expression: –



The utmost importance of recording the error and rate of a chronometer can be appreciated when it is realised that in the event of a radio failure during the course of a long ocean passage, without such a record the navigator would quickly lose track of the error of his chronometer, the timing of his Astronomical observations would become inaccurate and serious errors would occur in his position fixing. For this reason, the chronometer error and rate should be recorded daily in a Chronometer Rate Book or Chronometer Rate Card kept inside the chronometer locker.

This should be drawn up and filled in similar to the example RATE RECORD CARD.

It will be seen that the Chronometer Rate Record also shows the ambient temperature in the chronometer locker. Taken from a maximum /minimum thermometer each day at the time the chronometer is wound. 

This is because as was explained earlier in the chapter, the rate of a chronometer is closely related to temperature. In the above example, the vessel is obviously entering a tropical climate and the daily rate of the chronometer is decreasing as the standard temperature for a compensated chronometer is approached.

As already stated, it is desirable to check the chronometer daily so far as possible by radio time signals but the vessel in the example above was apparently unable to do so between 02nd May and 07th May, and between the 08th and 11th May. The accumulated rate between 02nd and 07th May was 7.5 seconds in exactly 5 days so the daily rate was: –

For simplicity of calculating the daily rate it is obviously more convenient if the error is taken from a radio time signal at the same G.M.T. each day, but where this is not possible always work in days and decimals of a day (to two places) and reduce all times to G.M.T. to avoid the difficulties of using Zone Time or crossing the ‘date line’. At 1500 G.M.T. on 08th May the error in the record above was 2m. 05s fast and at 0900 G.M.T. on 11th May the error was 2m. 07s fast. making an accumulated rate of 2.0 seconds in 2.75 days. so the daily rate was:

Time signals may become unobtainable during a long ocean passage due to radio failure. Providing that careful records have been kept (as described above), the error of the chronometer can be calculated from the daily rate and the last know error. The calculated error can then be applied to the observed chronometer time to give G.M.T. This is done by writing down the time by chronometer and applying the last known error, adding if slow but subtracting if fast. The accumulated rate is then found by reckoning up the number of days and decimal parts of a day which have elapsed between the date of the last error and the time by chronometer. and multiplying them by a daily rate. This accumulated rate is then applied to the chronometer time (already corrected with the last known error) adding if the chronometer is losing, subtracting if the chronometer is gaining, to give the required G.M.T.

The required G.M.T. for the observed chronometer time on 04th Jun was, therefore, 3d. 21h. 21m. 51s. (Note that when reckoning the number of days for the accumulated rate, the Greenwich date in the chronometer time must be used. as this may be different to the date at the vessel as in this example.).

It may be necessary in some cases to find the daily rate from two previous errors. If both these errors are slow or both fast, subtract one from the other, but if one is slow and the other fast, add them together to find the accumulated rate between the errors, dividing this by the time between the errors to find the daily rate. The procedure is then the same as in Ex. No. 1 above.

The required G.M.T. on 29th May on this vessel is, therefore, 29d. 4h. 19m. 44s.

The required G.M.T. on 08th Oct on this vessel is, therefore, 7d. 18h. 11m. 18s.

Self-Test Exercise – Find the G.M.T. in each of the following:

  • 19th Jul – time by chronometer 20d.4h.21m.36s. – lm.35s. slow @ 12:00 G.M.T. 14th Jun gaining 2.5 sec daily.
  • 05th Jan – time by chronometer 5d.16h.13m.55s. – 0m.58s. fast @ 12:00 G.M.T. 20th Dec losing 1.5 sec daily.
  • 22nd Nov – time by chronometer 22d.11h.45m.36s. – 25th Jul 04m.36s. slow @ 12:00 G.M.T. and on 08th Sep 02m.3s. slow @ 12:00 G.M.T..
  • 08th Mar – time by chronometer 8d.17h.44m.36s. – 03m.46s. fast @ 12:00 G.M.T. 22nd Dec and 02m.49s. fast @ 12:00 G.M.T. on 29th Jan. (assuming it is not a Leap Year).


(a). 20d. 04h. 21m. 42s. G.M.T. 

(b). 05d. 16h. 13m. 21s. G.M.T. 

(c). 22d. 11h. 43m. 24s. G.M.T.

(d). 08d. 17h. 42m. 44s. G.M.T.

error: Content is protected !!