level of difficulty

Keeping the time

keeping the time is crucial to have good results from celestial navigation. Because the Earth is in constant rotation, moving 15░ each hour, a four seconds error in the time figure will result in a position error of up to one nautical mile.

Longitude is the coordinate affected by time errors. In fact, if you don't have the correct time, you can't calculate your longitude using celestial navigation. This problem challenged navigators and scientists (even Isaac Newton! ) for centuries.

By the end of the 17th century it was generally accepted that no watch could *ever* be build that, taken to the sea conditions, could perform well enough to be used as a navigation tool. Scientists hoped they could find a way to adjust navigation watches using the Moon position or the eclipses of Jupiter satellites. That was their best bet to help seamen.

Harrison's masterpiece: the H4

Fortunately a man called John Harrison proved they wrong. By designing and building a series of precise timepieces, with innovative mechanical features that corrected the effects of boat movement and temperature variation, Harrison succeeded in creating a new and important navigation device: the marine chronometer.

Astronomers did not believe the chronometer was feasible and even after a successful test, they were not convinced that it could be mass produced. After a long argument, Harrison end up winning the all important "Longitude Prize", for solving the greatest navigation problem of his time: one that have destroyed entire fleets of ships.

A good book about the life of Harrison is
"Longitude: the True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time"

by Dava Sobel (ISBN 0-14-025879-5 )

Links: Harrison and the Longitude Problem | Wikipedia Entry | TV Documentary

The Lunar calculation

While Harrison worked his mechanical puzzles, astronomers were figuring their own method to adjust watches in blue waters. The eclipses of Jupiter satellites could be used, as Galileu proposed two centuries earlier, but they found it was difficult to observe such small objects from a rocking boat, even with good telescopes.

A better option was the Moon. When compared to the "fixed" star background, the Moon is the fastest moving sky object (faster than all the planets and the Sun). The Moon travels more or less 12 degrees each day ( or 30' per hour).

While the Moon position can be used as a clock, it requires very precise sextant measurements and even with the best equipment, the precision is limited. But it was the best thing they had.

Unfortunately, the Moon also has a very complicated movement, affected by the gravity of the Sun and Earth. The existing moon tables were not good enough for navigation purposes. So, astronomers set to improve the Moon tables, and hopefully solve the longitude problem once and for all (not to mention the money prize).

In the end, the Lunar calculation and the tables needed for it were perfected and used, but the chronometer proved to be much more convenient and precise. For more details on the Lunar Calculation click here.

Link: Lunar calculation (Centennia Software)

Time keeping

Even the best mechanical chronometer is affected by boat movement and temperature. So they were kept near the boat keel where ship rocks less. Larger ships usually carried numerous chronometers, to improve reliability. The Beagle (the ship Charles Darwin went around the globe in his famous survey travel ) carried 22 chronometers on board.

They were wound once a day, at the same time, by the same person and with the same number of turns. The expensive chronometers would remain on a safe place and the time would be copied to a smaller, cheaper watches for day-to-day use. All to make sure that these devices behaved in a predictable way.

Today quartz watches are much more reliable and predictable. They are not affected by boat movement and little affected by temperature. A good quartz watch can be used for celestial navigation. But we still have to use some old techniques:

  • Having two or more watches is necessary, for reliability. At least one of them should be kept in a safe place, protected from the sun and water.
  • Know your watches (and the watches of other crew members). A watch behavior should be observed well before taking it to the sea as a navigator watch. You must know how much time the battery will last and how the watch error increases over time, compared to the official Universal Time (UTC).
  • Calculate the watch error change rate every two weeks, starting at least 3 months before the actual navigation use. Most electronic watches have a stable error change rate. I use a TIMEX that adds 4.5 seconds every month.
  • Keep a page in the navigation log book for each watch, were information about battery changes, time adjustments and watch error change pace are kept. This page can also be used calculate the current watch error.
  • Have spare batteries on board.
  • Forget the computer built in clock. These are very bad clocks and behave erratically. Use a good quartz watch instead.

Time from the Internet

As I said, the computer clock is less reliable then a quartz watch. But you can use your computer to obtain the accurate time, as long as it is connected to the Internet. There are several reliable time sources in the Internet. These are accessible using SNMP, Time and Daytime protocols compatible clients.

A good time keeping program is Dimension 4. It can be installed as a service and will automatically keep the computer clock adjusted while connected to the Internet. It is freeware. Reference time sources include the US Naval Observatory and several universities and research centers. Check Dimension 4 web site.

An interesting URL is nist.time.gov. This is a Java applet with the official U.S. Government time. Click the UTC link to get the GMT time. It's said to be within 0.6 sec, depending on the latency of your internet connection. Requires no special client software.

Other time sources

Correct UTC time can be obtained in a number of sources.

  • There are short wave band radio stations that broadcast the time continuously. The Hydrographic Office publishes a list of these stations worldwide (H.O. 117).
  • TV and radio networks usually have good time keeping, to coordinate their network activities.
  • Large vessels always have the correct time.
  • GPS receivers are also a good time source, as they receive time data from very precise atomic clocks on the GPS satellites.


ęCopr 92-2012
Omar F. Reis - All rights reserved