It is said that Victorian sailors sometimes swore by ‘Almighty God and an Admiralty Chart’, a tribute to their trust in the work of survey vessels. Captains of merchant ships sailing from Britain to trading ports all over the world had always wanted accurate charts of their route, complemented, if possible, by comprehensive sailing instructions. Several private publishers, mostly based in London, attempted to meet this demand. A late C18th example was Robert Sayer’s ‘East India Pilot’, a compendium of more than 200 charts showing the way from the Thames to the ports of China and Japan. Information for privately published charts came from various sources including plans by sailors who knew the routes, the maps of competitors or surveys sponsored by the French government. Inevitably some charts were much more accurate than others and the great cartographer and explorer Captain Cook blamed navigators who were prone to flights of fancy for much of this unreliability, accusing them of including coasts they had never seen and soundings they had never taken. Compounding the problem was the difficulty of navigation at sea, particularly in ascertaining where a ship was in regard to longitude.

Pinpointing a place on a map or a chart that has indicators of latitude and longitude is straightforward.  A north or south point of latitude indicates a place north or south of the equator. Concentric lines of latitude lie between the equator and the north or south poles (1) and measurements are done in degrees, minutes and seconds. The measurement between degrees is always 70 miles. Calculating latitude, done by astronomical observation, was never as much of a problem as calculating longitude.

Today an east or west point of longitude indicates a place east or west of the Greenwich Meridian, a meridian being an imaginary line that circumvents the world through the north and south poles. Even in Ancient Greece it was recognised that if something regular and unchanging could be observed from two places that had a known time difference, it would be possible to work out the longitudinal distance between them. The obvious source of this information was the sky with its endless and repetitive movement of the sun, moon planets and stars. In the C15th, at the start of the great era of European sea explorations it was thought that if a table, or almanac, of predicted astronomical movements was carried by an adventurous sailor then it might be possible to work out longitude weeks or months into the voyage. Such an almanac would need a base line of course, a point or line from which longitudinal distances could be measured. The Ancient Greeks opted for one based on the Canary Islands, the edge of their known world and this continued to be used well into the post-medieval period. The French, leaders in cartographic development, used Paris, whilst the British preference for Greenwich only came to international prominence after Nevil Maskelyne’s ‘Nautical Almanac’, a table of predictions based on lunar distance readings taken at the Royal Observatory, Greenwich, was published in 1766. The longitude measurements that this almanac facilitated were based on a meridian deemed to run right through the observatory and the publication became so popular amongst European navigators that, for a time, even the French official almanac reproduced them.

Other imaginary lines run on either side of the Greenwich Meridian (2), dividing the world into 360 divisions of one degree each. Thus there are 180 degrees west of Greenwich and 180 degrees east. The measurement of longitude is in degrees, minutes and seconds, but the distance between each degree will depend on how near to the poles they are. As there are 1,440 minutes in a day it takes the sun 4 minutes to apparently travel from one meridian to another and, consequently, 12 o'clock noon on one meridian will seem a little earlier when viewed from the meridian to the west and a little later from the meridian to the east. When the Saracen was being built in Plymouth that town, like all others in England, kept its own time, based on local perception of the position of the sun. When it was noon in Woolwich, which is adjacent to Greenwich, it was only sixteen minutes to twelve in Plymouth, so the west country shipwrights had to wait a little longer for lunch!

Successful use of any almanac would demand precise observation of heavenly bodies and an accurate clock. In 1610 Galileo Galilei made an important breakthrough when, using a newly developed telescope, he found that he could see four of Jupiter moons. At least one of these was eclipsed each night, such eclipses appearing to occur at precisely the same moment from wherever they are observed on earth. This gift from the solar system offered a chance of enhanced accuracy for a longitudinal surveyor, but it was not until the late C18th that a comparable major breakthrough in timekeeping took place. John Harrison, son of a Yorkshire carpenter, spent a working lifetime developing an instrument accurate enough to aid the calculation of longitude through all weathers and sea conditions. When Captain Cook sailed to the Pacific in 1772 he took a replica of one of Harrison’s clocks, which was small enough to be held in the hand, and a copy of Maskelyne’s ‘Nautical Almanac’. The voyage lasted almost three years and near the end Cook wrote that the timepiece had;

exceeded the expectation of its most zealous Advocate and by being now and then corrected by lunar observation has been our faithful guide through all the vicissitudes of climates.

So, by the end of the C18th, the problem of accurately calculating longitude had been finally resolved. The chronometer was the timepiece which evolved from Harrison’s clocks.

Chapter 5 - Exporting free trade, importing tea

Back to Introduction

Chapter 4 - The problem of  longitude



Flamsteed House, the Royal Observatory

The Thames at Greenwich

Soochong, Shoguns and the Saracen’s surveys

When London Became An Island