Message in a Bottle Third Edition
PAGES
- Front Page - Message from the Editor
Page 2 - Portside Perusal: Global Event Gossip
Page 3 - Meliva vs Clockwork
Page 4 - Navigational Aids
Page 5 - Meet The Crew & Ship Stealing
Page 6 - Historical Lookback
Page 7 - Special Feature: Tall Tales of William One Eye
Navigational Aids:
We don’t doubt that you’re an expert modern-day navigator, but do you know your cross staff from your astrolabe?
We’re about to test your knowledge on nautical navigation throughout the ages!
How did we navigate before ECDIS? (Electronic Chart Display and Information Systems)
Over the last five hundred years, dramatic advances in navigational science and technology have altered the navigator’s work and methods. Most recently and most significantly following the introduction of ECSDIS That said, the navigator’s basic task remains constant: to monitor where the ship has been and where it is now, and to plan where it will go next on its voyage.
Since ancient times it’s been possible to make an estimated calculation of latitude. For hundreds of years however, there were two very serious problems. In earlier years, this was limited understanding of direction – even following the birth of the magnetic compass, navigators had a very poor idea of direction or accuracy, since the concept of variation was not understood till many years later.
The second problem was even more important. In the absence of an accurate time keeping device such as a chronometer, a calculation of longitude was just not possible. It’s not then surprising that early navigation was problematic and often resulted in the navigator becoming lost, especially when out of sight of land.
The first ever clock leads to navigation of the pacific
Without doubt, the most famous pioneer of the navigation revolution was Captain James Cook who navigated the expanse of the Pacific thanks to the technical genius of John Harrison. In 1759, Harrison devised the first ever clock to keep the time exactly – in any variation of climate.
Keeping exact Greenwich Mean Time, Harrison’s clock enabled sailors to determine how many hours there were before or after that time and it was then possible to determine their longitude (degrees East to West position) on the globe.
Lead line
Perhaps the oldest navigational tool on record originating in Egypt, the lead line is a measuring tool designed to assess the depth of the water and take a sample of the ocean floor. The lead line comprised of a hollow weight made of lead. In the middle of the weight sat a ball of animal fat which collected material from the ocean floor. This is the first recorded method of using depth and seabed material to determine location.
Compass
We’re pretty sure you got this one, but do you know the origins of the compass?
Originating in the Han dynasty China between 300 and 200 BC, the compass uses the magnetic poles of the earth to find magnetic north. In later years, compasses used iron needles, which were magnetised by striking them with a lodestone. By 1300 in Medieval Europe and the Islamic world, dry compasses started to emerge, replaced by liquid-filled magnetic compasses in the 20th century.
Compass rose
Still used today, a compass rose is a design on a compass, map, nautical chart, or monument used to display the orientation of North, East, South, and West, as well as their intermediate points. Before compass roses were used on maps, lines were drawn from central points. These lines were hard to follow since there were so many lines intersecting each other on one map. The rose design was typically drawn in a way that made it easier to follow the directional lines.
Log line
The log line is an old instrument for measuring the speed of the ship. It consisted of a flat piece of wood which was weighted at the bottom edge to enable it to float upright in the water. The log was attached a long rope. The log line was wound on a spool so that it could be reeled out after the log was thrown into the water at the aft of the ship. The friction of the water held the log in place as the ship sailed away from it. As the ship sailed away from the log, the sailors taking the reading would count the number of knots that passed over the rail in a period of half-a-minute.
Sand glass
With records dating back to the 14th century and with likely use before in ancient times, the sand glass is a primitive timepiece used to measure the time (usually 30 seconds at a time) on a voyage, or on a specific navigational course. Sandglasses came with some major drawbacks, most notably that design and environmental factors affected the rate of the sand’s flow, giving inaccurate measurements. By the 19th century, they were replaced by more reliable & accurate mechanical timepieces and technology.
Cross staff
The cross staff, also known as the early sextant, consists of a wooden rod with a sliding crosspiece used for measuring the altitude of a star. It is thought to have originated in the 14th Century and was originally used as a land surveying instrument.
Nocturnal
Using the position of the stars to tell the time, the nocturnal was particularly primitive in design. One ring was set to the date, Polaris was sighted through the hole in the centre and the arm was moved into a position to align with pointer stars. Where the arm crossed the marked ring, the time was read.
Quadrant
An early instrument for measuring altitude of celestial bodies, consisting of a 90° graduated arc with a movable radius for measuring angles.
Astrolabe
The astrolabe was an inclinometer used to determine the latitude of a ship at sea by measuring the sun’s noon altitude (declination) or the meridian altitude of a star of known declination.
Traverse board
Developed as early as the 1500s, the traverse board comprises of a small board marked with the four points of the compass & eight holes bored at each point to represent each half hour period in a watch: this was used to peg the courses made by a ship in each half hour.
Backstaff
Invented in 1590, the backstaff is a navigational instrument that was used to measure the altitude of a celestial body, in particular the sun or moon. When observing the sun, users kept the sun to the back of them and observed the shadow cast by the upper vane on a horizon.
The sextant
The Longitude Act of 1714, offering the sum of £20,000 for the practical solution of the problem of the determination of longitude at sea to an accuracy of 0.5 degree, may have been the single greatest factor in establishing Britain as the world’s leading nation of navigators for almost a full century.
During the earlier years of the Board of Longitude, existing instrumentation was as inadequate for the accurate measurement of a Lunar Distance (the angle between the Moon and selected reference stars as the Moon moves against the stellar background) as was lunar theory for predicting lunar motion. The cross staff had come out of the East during the Middle Ages. The invention of the Davis quadrant (or back-staff) was over 100 years in the past. The simple quadrant and the mariner’s astrolabe were even less appropriate. Therefore, when John Hadley presented his new, doubly-reflecting quadrants before the Royal Society in the year 1731, a new order of instrumental accuracy was established. The sextant is a doubly reflecting navigation instrument that measures the angle between two visible objects.
Originating in 1758, a round brass frame with two wooden handles was developed to assist with navigation at sea. Named the reflecting circle, the tool allowed navigators to achieve superior accuracy through the use of a circle of 360 degrees, instead of the arc of 120 degrees as used by the sextant.
