Improved navigation

Latitude

Longitude

Latitude & Longitude

Latitude and Longitude: many of us find it difficult to remember which is which! However, if finding your way about the world is important, you will soon remember – or get lost! All of our Ordnance Survey maps use grid references which are based on latitude and longitude. Use your local OS map to find the grid reference of your home.


Latitude

Knowing your latitude tells you how far north or south you have travelled.

Imagine a ladder: the rungs that you tread on are lines of latitude and the long poles to which the rungs are fixed are the lines of longitude, the ground is the equator and the end of the ladder is the north pole. To find out how far you are away from the equator you simply need to count the number of rungs between you and the ground.

How does this help a sailor? The lines of latitude are imaginary but accurate. The North Star is the one star in the sky – in the northern hemisphere, that does not change its position and this has been recognised by sailors for hundreds of years. If you can see the North Star you can find your latitude. If you are at the equator, the star will be just above the horizon, if you are at the North Pole it will be directly above your head. It is the angle between the star and the horizon that indicates your latitude. The same principle holds true for using the sun during the day except the angles are the other way around.

To help you understand this, consider that you are climbing your ladder to fix a light bulb. When you are standing at the base of the ladder – at the equator, the bulb will be directly above your head and you must look up in order to see it. However, when you have climbed to the top of the ladder – our equivalent of sailing to the North Pole, you must look sideways to see the bulb. The angle of you vision tells you how high you have climbed.


Navigational instruments for latitude

In order to find latitude, sailors have used several methods and special instruments. Christopher Columbus recorded having used a quadrant on his journey to discover America in 1492 AD. The quadrant divides the sky into four sections and is used to measure the angle between the horizon and the star or sun. This later evolved into the sextant and octant.

The quadrant lacked accuracy and this led to the invention of the cross staff. A horizontal stick would be lined up with the horizon and a vertical slider would be adjusted along the length of this until the sight of the sun was in line with the top of the slider and the end of the stick. The ratio of the slider's height to the length of the stick between the eye and the slider could be converted into an angle which indicated the sun’s height.

The back staff was a different version of this which avoided having to look directly into the sun, a practice which could seriously damage the vision of the navigator!

A sailor's quadrant

A sextant


Questions

It's daybreak and you are sailing with the sun directly to your left. In which direction are you sailing?
The Polynesian sailors think that the position at which the sun rises is the most important information for their navigation. Why is this?
Would Christopher Columbus have been able to use the North Star to navigate on his journey to America?
Captain Cook discovered much of the land around Australia. Would he have found the North Star useful?
Why do you think that the equator is given the value of 0 degrees of latitude?
Another navigational device uses the length of shadow to indicate the sun’s angle. At midday, would the shadow cast be longer or shorter if you were more towards the North Pole than the equator?

Finding your latitude by using celestial bodies depends on three things; being able to see the sun or star, being able to see the horizon and making sure that you are lined up accurately with both. What problems exist on board a ship at sea that may make these sightings difficult?
a) Moving boat
b) Waves making the boat move up and down
c) Fog, haze, clouds or rain
d) Difficulty distinguishing between the sea and sky at night.



Longitude

Knowing your longitude tells you how far east or west you have moved.

Finding longitude is far harder than finding latitude and this has been the cause of countless mistakes in navigation. In the times when the Earth was being explored by the Europeans it was not uncommon for ships or even fleets to go missing because the did not know their longitude. In fact, it was so important to be able to find longitude in 1714 King of England announced a reward for £2000 [Worth more than £2,000,000 in today’s money.] for the first person to solve the problem. A special committee was formed to test any ideas presented. This was based at the Royal Observatory in Greenwich, London. Apart from some of the usual crazy ideas suggested, the attempts to find solutions fell into two camps; those who favoured celestial methods and those who favoured accurate timing.

[Click here to find out more about the history of the Royal Observatory Greenwich]

The Royal Observatory in the late 16th Century

It's all a matter of timing...

Why does timing matter? If you can imagine looking down on the Earth from above the North Pole you will see a circle. You know that a circle is divided into 360 degrees. You also know that it takes 24 hours for the Earth to spin once, therefore, in one hour it will have turned 15 degrees. If you know your latitude, which is not a problem, you can work out how far you have travelled east –west, provided you know two times – the time it is where you are and the time it is at your starting point. As the earth has a circumference of about 40 000km at the equator, each hour or 15 degrees represents about 1600km.

Your challenge...

Can you sketch a diagram to illustrate the concept described above?

 

The only clocks at the time were based on the pendulum. Why do you think that it was difficult to know the time at the starting port?

One determined individual, John Harrison, a carpenter, pursued his ideas about how to build an accurate timepiece for the whole of his life. Eventually, he built the chronometer (pictured right) – an absolute masterpiece of design and engineering, this met the rigorous standards and he was awarded the prize. [Not without a few problems – read the book ‘Longitude’ by Dava Sobel.]


Harrison's chronometer

If you can find your latitude and longitude you know
where you are at any point on the Earth’s surface.
With an accurate map, you can find you way to any port avoiding all known obstacles.

Questions

Why do you think that Greenwich is given the longitude of 0 degrees?
Harrison’s chronometer was jealously guarded by the British. The captain of each ship that carried one had to destroy it if the ship was in danger of being captured. Why was this?
If you are 15 degrees to the west of Greenwich you are not 1600km away. Why is this?




Find out more about navigation:

Introduction
Dead reckoning
Early navigation
Improved navigation
Modern navigation

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© NOCS
February 2007