The Chronometer

Chronometer Error and Chronometer Rate

Greenwich Mean Time is invaluable to the navigator and the chronometer is the instrument which provides it. But like any instrument, the chronometer is subject to error. Routinely, candidates for Ocean and Near Coastal deck credentials are required to determine Chronometer Error and Chronometer Rate. This week we will do both.

Chronometer Error (Stopwatch)

You are taking a time tick using the 1400 signal from Kekaha-Kauai, Hawaii (WWVH). You hear a series of 1-second dashes followed by a 9-second silent period, then a long 1.3-second dash. At the beginning of the long dash, you start your stopwatch. At the time the chronometer reads 02h 02m 15s, the stopwatch reads 1m 37s. What is the chronometer error?
A. 1m 28s slown
B. 1m 12s fast
C. 0m 22s slow
D. 0m 38s fast
Answer: D

There is an assumption in this question and in almost all other questions on chronometer error that the radio and the chronometer are not in the same place. In fact, frequently they aren’t. Someone looking at the chronometer may not be within earshot of the radio.  Determining the chronometer error will require transporting certain information from the radio to the chronometer. This may be done in a couple of ways.  One way is to use a stopwatch.  As soon as the 1400 signal sounds from Kekaha, Kauai (WWVH),  the mariner starts a stopwatch.  Then he or she walks — slowly, quickly, the use of the stopwatch makes the pace inconsequential — to the chronometer, at which point the stopwatch is stopped and the reading on the chronometer recorded.The time on the stopwatch is the interval between GMT 1400 and GMT when looking at the chronometer.  Adding the stopwatch time to the 1400 GMT gives the GMT equivalent to whatever is on the chronometer.  The difference between the two values is chronometer error, named fast or slow depending upon whether the chronometer is behind (slow) or ahead of (fast) the calculated GMT. 

Here add the 1m 37s it took to get from the radio to the chronometer to 14-00-00 to find the GMT.  Compare that GMT (14h 01m 37s) to the chronometer’s reading of 14h 02m 15s to find that the chronometer is running fast by 0m 38s.

Chronometer Error (Comparing Watch)

The second way of transporting GMT to the chronometer is by means of a comparing watch, an extra watch that keeps “rough” GMT.

You are off the coast of Mexico and are taking a time tick for 1800. At approximately 1754, you hear the preparatory signal "VVVV de XDD" from the time signal station. Then you hear a series of 1 second dashes followed by a 9 second silent period and then a long 1.3 second dash. At the beginning of the long dash, your comparing watch reads 06h 00m 07s. When compared to the chronometer, the comparing watch reads 06h 01m 24s, and the chronometer reads 05h 59m 23s. What is the chronometer error?
A. 0m 07s fast
B. 0m 37s slow
C. 1m 54s slow
D. 1m 17s fast
Answer: C

In this question, the comparing watch reads 06h 00m 07s at GMT 1800.  Adding 12 hours to the comparing watch on the assumption that like the chronometer it is a 12-hour clock, gives a comparing watch time of 18h 00m 07s at GMT 1800.  The difference between these two times is 07 seconds.  The comparing watch is 7 seconds faster than GMT.  Armed with this information, the mariner could take a walk, take a bath, take a nap knowing that when he finally gets back to the chronometer, Greenwich Mean Time is the time on the comparing watch MINUS 7 seconds.

Thus when the comparing watch reads 06h 01m 24s (18h 01m 24s), GMT is 18h 01m 17s.  If the chronometer at this time reads 17h 59m 23s,  the chronometer is 1m 54s slow.  The answer is C.   

Chronometer Rate

On 23 July, you take a time tick using the 0900 GMT Cape Town broadcast. You hear a repeating series of 59 dots followed by a dash. At the beginning of the fifth dash, you start your stopwatch.The chronometer reads 08h 39m 16s at the time the stopwatch reads 01m 42s.The chronometer error at 0900 GMT, 22 July, was 22m 24s slow. What is the chronometer rate?
A. 00m 02s losing
B. 01m 02s gaining
C. 22m 24s losing
D. 22m 26s slow
Answer: A

By working out the chronometer error for 23 July (22m 26s) and comparing it with the chronometer error for the day before (22m 24s slow), we can determine that the chronometer is losing 2s a day.  This is the chronometer rate 

When working this problem and most other problems, disregard “You hear a repeating series of 59 dots followed by a dash.  At the beginning of the fifth dash, you hear a 0.4-second dash followed by a series of dots, noting that the 29th and the 56th to 59th dots are omitted. At the start of the following 0.4-second dash (which is followed by an 8-second pulse).” These are the time signatures for the ticks at various stations.  In the previous question, for example, the time signal came from Mexico.  But for purposes of the exam, these questions might just as easily read “You hear four calling birds, three French hens, two turtle doves and at the quack of the partridge in a pear tree,” you start the stopwatch or check your comparing watch.  

Of course, partridges don’t quack.


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