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Stars and Clocks:  Navigation in Baudin’s Time

Today, the world seems very small compared with the way in which it was thought in the past.  The ease of international travel and communication mean that it can be difficult, especially for the younger generation, to conceive of a time when ships could be lost simply through poor navigation.

Long ago, for the crew of a ship out at sea there was no way of contacting anyone else.  They were limited to actually seeing another ship in the distance and using a telescope to try to determine its country of origin and intent from the type of ship it was and the flags it was flying.

Just as importantly, or even more so, knowledge of their exact location at sea, or even when on land away from well-established settlements, was hardly simple.

We can quote our location on Earth by using two quantities: our latitude and our longitude.

It’s worth mentioning at this point that the Earth is not a perfect sphere.  Its shape is called an oblate spheroid.  Such a shape is a slightly flattened sphere: for the Earth, its polar diameter is slightly less than the equatorial diameter.  Among other things, this leads to a distinction between geographic latitude and geodetic latitude. There is also another type called astronomical latitude, which is affected by local topography.

However, for the purpose of a simple discussion of latitude and longitude, I shall treat the Earth as a perfect sphere.

Latitude is a measure of how far a location is north or south of the equator, and it is measured as an angle subtended at the centre of the earth between the equator and the location in question. Zero latitude is exactly on the equator and 90 degrees north or south are at the north or south pole, respectively.

Longitude is a measure of how far a given location is around the Earth, from the line called the Greenwich Meridian which joins the poles and passes through the Greenwich Observatory in England.  It has a range of a full 360 degrees – 180 degrees both west and east of Greenwich.

The Greenwich Meridian is also called the prime meridian of the world.  However, unlike the equator, about whose location nobody can argue, there is no physical aspect of the globe that determines an automatic zero line for longitude.  This decision is, basically, a political one, and the Greenwich Meridian was finally officially chosen as recently as 1884, even though it had been in use for a long time.

Another meridian that had long been in use was the meridian passing through Paris, and naturally the French and the English had a hard time agreeing on the one that should be used as the world’s zero of longitude.  Even after the famous 1884 decision, the French regarded Greenwich mean time, on which the world’s time systems were based, as Paris mean time, retarded by nine minutes and twenty one seconds.  Interestingly, this definition lasted until 1978.

The exact location of the zero line was not of major consequence.  Whichever definition one used, provided one knew how far east or west of the line one was located, one’s longitude would be known.

So the big question is: how was it possible to know one’s latitude and longitude?  It turns out that latitude is really quite easy to find, but knowing one’s longitude was very difficult indeed.  And centuries ago, it was considered to be one of the great scientific problems of the time.  As European nations – in many cases at war with each other – sent their ships on voyages to different parts of the globe to seek new lands and treasures, it was increasingly important for those on the ships to know exactly where they were at any given time.

And this is where both astronomy, and clocks, come in to the picture.

As is well known, the stars we see in the sky can depend on our latitude.  For example, from Tasmania we never see the group of stars known as The Plough in England (also known as The Big Dipper in the USA), and from England the Southern Cross never rises above the horizon.

Most importantly, the maximum angular height above the horizon for the Sun and nighttime stars depends on one’s exact latitude.  The Sun’s own position in the sky changes over the course of the year, but these variations are well known – so by measuring the Sun’s maximum height in the sky and knowing the date, the latitude can be deduced.

The situation is quite different for longitude.  The view of the Sun or the stars from a given longitude will be duplicated at all longitudes around the Earth over the course of a rotation.  For example, the view of the sky over Sydney is the same as the view of the sky from a point just north of Adelaide about 51 minutes later, and it is duplicated again at Esperance in WA after another 66 minutes.

On the open sea, there was therefore potential for major errors in longitude, even though latitude could be determined relatively easily.

The longitude problem, as it became known, was a major reason for the establishment of the Paris and Greenwich observatories, which were completed in 1671 and 1676 respectively.

As everyone knew at the time, finding one’s longitude would have been a simple matter if one could compare the view of the sky one saw with the view at a known longitude.  It would then be easy to work out the offset.  For example, if a given star were at its highest point at Greenwich at a particular time, the star would still be 30 degrees offset from its highest point at a longitude 30 degrees west of Greenwich.

So, knowing the time at Greenwich, or Paris, or any other known location would enable one to calculate one’s longitude.  We can do this easily today as we travel around the world, and for those not too sure, the flight crew on a passenger aeroplane normally remind the passengers to alter their watches if they were still set to the time back home.  But in an age when timepieces did not work well enough at sea, how was one to know the time at home?

In fact there were two astronomical methods of finding the time at a known longitude.

One method, suggested by Galileo, was to make telescopic observations of the four bright satellites of the planet Jupiter.  They appear as points of light that move around and around the planet in orbits taking known amounts of time, and so by examining the current pattern made by them it is theoretically possible to know the time.  It was only after Galileo’s death that the method was used, and even then, only on land.

The Jupiter method worked reasonably well, and did improve people’s knowledge of the longitudes of various places.  It wasn’t always a happy thing: it was discovered during the reign of Louis XIV that France was smaller than expected, prompting him to comment that he was losing more territory to his astronomers than to his enemies.

Another method involved the Moon. Because the Moon orbits the Earth, it moves appreciably against the backdrop of the stars, appearing to move its own diameter in about an hour. So by comparing the position of the Moon with the background stars, it was theoretically possible to work out the time in Greenwich or Paris.

The ‘Lunar Distances’ method is quite complex because one needs to have good tables of the expected positions of the Moon, and even more complex because the Moon was relatively close to the Earth and the Earth’s own diameter affected the observations, because the Moon was viewed from slightly different angles as the Earth turned.  But it was a method strongly supported by the fifth astronomer Royal, Neville Maskelyne, and developed by him in the 1760s.

Neither method was satisfactory enough to give sufficiently accurate longitudes, and the Board of Longitude in England offered a 20,000-pound prize to someone who could solve the problem.

One of the important prompts for doing this was the fact that ships could be lost at sea if their navigators made errors in their assumed locations.  A famous example was the loss in 1707 of 1550 sailors when a fleet led by Sir Cloudesley Shovell foundered on the Isles of Scilly.

Another was Commodore George Anson’s disaster in trying to find the Juan Fernández Islands in 1741.  His error in longitude caused considerable delay in locating the island, and in the meantime many men on the voyage passed away.  This led to a redoubling of the efforts to solve the longitude problem.

Eventually John Harrison, an English clockmaker, after several attempts, made a clock that worked well enough at sea to keep the time properly, so that the time back home would always be known, allowing navigators to make observations of the stars and work out the offset.  Even a clock unexpectedly losing or gaining as little as one minute over a period of months on a voyage would mean an error in longitude of a quarter of a degree, which can be nearly 30 kilometres.  The main point was that a clock need not be counting time at the correct rate: it needed to be consistent, with a constant rate of losing or gaining that was well known so that a correction could be applied.

Harrison’s great achievement was the clock called H4, which finally settled the issue after it was given thorough tests beginning in 1761.

As is famously known, the Board of Longitude did not give Harrison his prize, instead giving him incremental payments for his work.  It was only after King George III intervened that the payment was topped up.

And so we reach the time of great explorers such as James Cook and Nicolas Baudin.  By their time it was quite normal to be carrying chronometers on their voyages.

Baudin carried four chronometers and two sextants.  It was a good idea to have this kind of redundancy, in case one of the instruments performed poorly or was damaged.  The clocks needed to be wound according to a specific set of instructions, and if this was not done, or performed badly, the clock could become useless.  The clocks were their link with home.  Looking at the clocks, they knew the time in Paris just as looking at your watch on an international flight can tell you the time back home if you have not made any adjustments.

It can be said that through the late 1700s and early 1800s, it was the period in which, through the use of the marine chronometer, navigation blossomed.  Improved chronometers allowed for more accurate positions to be known.

There was no doubt, however, that a clock on land, being very stable, would potentially still fare better than a clock at sea, and it was also important for ships to check their clocks when in various ports.  To this end, many places had time balls that would be visible from a ship in port. It was able to drop down a shaft and this would happen at a specific time so that ships in the harbour, watching the ball drop, could check their timepieces.  You can still see one of those at the Sydney Observatory, and there are several others.

Today, our best clocks are more accurate than the rotation of the Earth itself for telling time.

Wouldn’t it be great to go back in time and show people like Baudin what we can do today?  A satellite-based GPS device shows our latitude and longitude almost instantly, wherever we are on Earth.  We even have maps based on our GPS position that we can see in our cars, and voices to tell us when to turn left and right when we are finding our way through a city.

Perhaps such a device in Baudin’s time would have a voice saying something like ‘In 1.719 days, change your course by 3.49 degrees.  You will be at the island 4.588 days later’.

However, there were no satellites, GPS devices, or electronics then.  Those were the great days of navigation!

Martin George, Collections and Research Manager

 

 

 

 

 

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