Education: navigation
The methods used to plot a course have changed dramatically over the centuries
THE skill of finding your way from one place to another is called navigation. Up until the 20th century, navigation mostly referred to the art of determining the position of a ship and plotting a course for it to a specific destination.
The word “navigation” is derived from the Latin words navis (ship) and agere (to drive). Today the word also refers to plotting a course on land, in the air and even in space. Here’s how navigation developed into the accurate science it is today.
SEAFARING IN HISTORY
In ancient times, seafarers navigated the ocean using primitive maps and determined their direction by observing the sun, stars and familiar landmarks. Until the 15th century, ships mostly kept in sight of land to help them navigate. It’s really difficult to work out where you are if you’re surrounded by ocean as far as the eye can see.
So, how do you calculate your position?
MAPS WITH COORDINATES
Ancient Greek mathematician Eratosthenes (276 BC-195 BC) first proposed a system of latitude and longitude. But it was ancient Greek astronomer, mathematician and geographer Hipparchus (190 BC-120 BC) who first suggested you could pinpoint your exact location by using longitude and latitude as coordinates (lines that cross at a specific point).
He proposed that a zero or prime meridian (vertical main line) should run through the island of Rhodes where he lived. He also suggested that latitude could be determined by simultaneously observing a lunar eclipse from various places.
Unfortunately, for this to work you needed an accurate watch – which hadn’t yet been invented.
MODERN GLOBES AND MAPS
Hipparchus was far ahead of his time and his idea later became a reality. By using a grid of longitudinal and latitudinal lines, we can pinpoint any location on Earth. Latitude lines run east to west (horizontally) across the globe, while longitude lines run north to south (vertically) across the globe. Though these lines are only imaginary, you’ll see them drawn on globes and maps as if they’re real.
The equator divides Earth in half horizontally into the Northern and Southern Hemisphere and all other latitude lines run parallel to it, which is why they’re called parallels. The equator’s latitude is 0°, and each latitude line horizontally measures north or south of it as far as 90°. This means the north pole (Earth’s northernmost tip) lies at 90°N (north) and the south pole lies at 90°S (south).
The prime meridian is the imaginary line that divides Earth into two vertical halves: the Eastern and Western Hemisphere. All the other longitude lines (called meridians) are the same length and meet at the north and south poles. This imaginary line runs through the Royal Observatory in Greenwich, London, in the UK, and so it’s also known as the Greenwich Meridian. It was chosen as the international prime meridian at a conference in America in 1884.
The prime meridian’s longitude is 0°. All other longitude lines run parallel to it as far as 180° west and 180° east. New York City in America
is situated at 74°W (west), while Durban in KwaZulu-Natal lies at 31°E (east). Time zones are also calculated according to where on the planet you are in relation to the prime meridian.
Degrees of longitude are divided into 60 minutes. For example, the longitude of New York City is 74° 42’ W (74 degrees, 42 minutes west). The longitude of Durban, is 31° 01’ E (31 degrees, 1 minute east). Each minute of longitude can be further divided into 60 seconds.
Today the coordinates are often converted to the decimal system. So then Durban is 31.0218° E. If you combine the latitude and longitude coordinates, you can pinpoint any place on the planet with great accuracy. Durban’s precise location is 29.8587°S, 31.0218°E. If you type in those coordinates into your GPS, you’ll end up nowhere else on Earth but in Durban.
LATITUDE AND THE MARINE CLOCK
Through the ages, seafarers could determine their latitude fairly accurately by using the stars, moon, sun and horizon as reference points to calculate their position. But longitude is harder to determine and was often just a rough estimate. The reason for this is you can determine your longitude coordinate only by calculating the difference between your starting point and your current position using the specific time at each position, and the early seafarers had no way of accurately determining what time it was. By the beginning of the 18th century, even the best clocks available still “lost” up to 10 minutes a day. That comes down to a calculation error of 242km or more.
The problem was finally solved in 1764 when English clockmaker John Harrison (1692-1776) invented an extremely accurate marine clock known as a chronometer.
British explorer Captain James Cook (17281779) used Harrison’s chronometer during his second voyage (1772-1775) to circumnavigate the globe as far south as possible to see if there were any undiscovered landmasses.
After the three-year journey the estimated longitude was off by just 13km. Thanks to this, Cook was able to update many shipping charts to help improve marine navigation.