I must go down to the seas again, to the lonely sea and the sky,
And all I ask is a tall ship and a star to steer her by.
– John Masefield
Human beings have had a long and important relationship with the seas and oceans of the world. The world’s waterways have been crucial for food, exploration, transportation, and for trade. In using the water as a means to get around, people have been faced with overcoming the difficulty of navigation at sea. The challenges of navigation at sea are obvious; on land there are generally clear landmarks that can be used for navigation. There are also villages, towns and cities with people in them to help you along the way. Once a ship is out of sight of land none of those things are available for navigation. Navigation at sea must be accomplished when the only sights available are the sea, the waves, the sky, and an occasional sea bird. How have people managed to do it? In this article we will explore the different ways that people have managed to navigate at sea.
From the early days of recorded history people have recognized that the sky was key to navigation at sea. About four thousand years ago, the Phoenicians were known as the most accomplished sailors in the world. They used primitive charts and observations of the Sun and stars to plot their location at sea and navigate. This is known as celestial navigation.
Celestial navigation is the art and science of using “sights”, or angular measurements taken between a celestial body such as the sun, the moon and the stars and the visible horizon. The sun is the most common celestial body used but navigators can use the moon, a planet or a navigational star. At any specific time a celestial body is located over a fixed point on the Earth’s surface (known as the Global Position or GP). The measured angle between the celestial body and the visible horizon is related to the body’s GP and the navigator’s position. Using computations that are called sight reduction, this measurement is used to plot a line of position (LOP) on a navigational chart. The LOP is a short segment of a large circle on the Earth that surrounds the GP of the celestial body. A navigator located in the circumference of that circle who is measuring the angle of the same celestial body at the same time would observe the body at the same angle above the horizon. Taking a sight on two or more celestial bodies allows the navigator to intersect those different angles and get a fix on their own position on the chart. This was the basic and original method of navigation at sea.
Early Improvements in Celestial Navigation:
There have been a number of instruments designed to expand on the principals of celestial navigation and improve the accuracy of the process. Arabic navigators developed the Kamal. A Kamal was a piece of rope or string that was attached to a transom. The navigator would hold the rope in his teeth and sight the Polaris star with the transom. Tying a knot would allow them to fix the position of Polaris at their home port. To return to home port, he would sail north or south as needed to bring Polaris to the altitude he’d observed when he left home and then sail down the right latitude to home. The astrolabe was another improvement in navigation at sea that was of Arabic design. The astrolabe was used to the find the time of the rising and setting of the sun and the altitude of the sun and selected stars. This improved the accuracy of sight readings. A similar principal was at work with mariner’s quadrants that are traced to the 1200’s, but became popular with European explorers like Columbus by the 1400’s. Quadrants are a quarter circle of wood or brass that span 90 degrees and are divided into whole degrees. It marks latitude and allows the navigator to take readings and trace back their position to their home port. The quadrant designed by Isaac Newton in 1699 was the ultimate in quadrant designs and led to the design of the octant and sexton.
Octant and Sexton:
The development of the octant and sexton was an important leap forward in navigation at sea. The octant is a reflecting instrument designed independently by John Hadley and Thomas Godfrey around 1730. It has a triangular scale that is divided into eighths (hence the name octant) and two mirrors. It allows a navigator to accurately sight the distance between two objects even with the rolling motion of the sea.
A sextant is a doubly reflecting instrument that allows the navigator to measure the angle between any two visible objects. It was designed by John Campbell in 1759 as an improvement on the octant. It determines the angle between the celestial body and the horizon. The determination of the angle (or the altitude) can be used to calculate the GP on the navigational chart. Sailors would take sightings of the Sun at solar noon and Polaris at night. It is also used for lunar navigation. The octant and sextant revolutionized taking sights and plotting longitude accurately. They replaced previous instruments and allowed navigators to take sightings and correlate them with Greenwich Mean time – a leap forward in the accuracy of navigation at sea.
Modern Radio Navigation at Sea:
Radio navigation allows the user to use radio frequencies to pinpoint a specific position on Earth. It is related to radio location and radio determination. Radio navigation functions by using measurements to and from electric beacons. In particular, radio navigation gives the user directions through bearing, radio phases or interferometry. It gives the user distances by ranging or measurements of travel times. It also assists the user partly through velocity by means of radio Doppler shift in radar. Advancements in technology have meant a corresponding rapid increase in the usefulness of radio navigation. Radio navigation systems use a type of directional radio antenna to pinpoint the location of a broadcast location on the ground. Conventional navigation techniques allow the user to make a radio fix. Radio navigation has been used at sea since the early days of radio technology and was improved dramatically throughout both World Wars. The process of miniaturizing electronics and the development of transistors further improved navigation at sea.
Over time the traditional forms of radio and celestial navigation have been largely replaced by satellite navigation systems. Satellite systems function much in the same way as traditional radio navigation by triangulating signals. These systems however must take the shifting position of the satellites they use into account, requiring computerization for calculations. Satellites have also led to the development of global positioning systems (GPS). GPS is currently the most accurate form of navigation that has been developed. It is useful almost anywhere on Earth and requires only a few satellites and relatively inexpensive technology to work. It has, therefore, steadily replaced most previous forms radio technology for use in navigation at sea.