Navigation

What is Navigation?

Throughout history man has built boats of many shapes and sizes. He has travelled oceans and seas, not always knowing his destination (he may have been an explorer) but, by and large has always known where he was on the surface of the earth. He determined this by watching the way of the sea, waves, smell, colour and temperature. He used to look up at the stars across the heavens to give him a course to steer by and to find his position.

Even today, with all the technology available, sea captains and ship masters still practise the art of celestial navigation (navigation by using stars).

Chinese Navigation

Can you believe that in the 3^rd century BC the Chinese were mapping the stars? Their manuscript star map of 940 AD is the oldest star chart from any civilisation. The oldest printed star chart in existence is of course Chinese, dated 1094. No other star charts are known to have been issued before the 15th century - amazing!!

By the first century AD the Chinese had discovered 14 020 stars, this is very nearly the amount that can be counted by eye (it would take a lot of night watches before I could possibly count half that amount...)

I wonder how many you can count where you live - can you name any of them?

Zheng He versus Precious Dragon

Magnetic Compass

This was a Chinese invention. On Zheng He's ship it was known as a "wet compass", that is a needle floating in water contained in a circular box with the compass points printed on the rim.

Special kinds of water were needed. A lot of care and attention was dedicated to the exact manner in which the needle floated. A lodestone was used to remagnetize the compass-needle.

Precious Dragon's compass is not dissimilar. The compass card with degrees from 0°-360° instead of points float in pure alcohol - to stop it from freezing in winter time. Our needle is not magnetised, but there are a group of magnets secured underneath the card.

The compass is the most important aid to navigation as this is what we steer the ship by. For example, we draw a line on our chart of where we want to go. On the chart are printed compass roses.

Using a special ruler - called "Captain Fields improved parallel ruler", we "move" the required course line to a compass rose on the chart to find the course to steer.

The Charts

We have 300 charts which cover the whole route that we are taking. Our oldest chart was made in the 19th century by sailing ships from the British navy. On the charts are lines of Longitude and Latitude from which we derive our position.

The charts show:

• The coastline of the area of where we are passing through
• All the lights & lighthouses - so we can sail safely at night without hitting reefs or rocks
• All the depths of the sea and the nature of the sea bed (bottom). This gives us the depth of water when we need to anchor.
• Shipping lanes - a kind of road for large ships - which we don't want to be close to.

Admiral Zheng He was not as fortunate as us - he had to make his own charts as he visited each new country - his cartographers would collect data which they later would incorporate into existing maps. Therefore by the time of his last voyage in 1433 Zheng He would have had charts that covered the area from the "Treasure Ship Yard" in Nanjing, China all the way to the Red Sea and the east coast of Africa.

Exercise

A brief explanation of Latitude and Longitude

Take an orange and a knife. Hold it so that the stalk points upwards. Take that point as being the north pole, the other end as the south pole. Now peel it - you will find inside that the segments divide the orange just the same way as lines of longitude run from pole to pole on the earth. There are more lines of longitude than segments in your orange - how many segments of orange have you got?

Now place your orange on a board with the side of the segments down and cut it into slices starting north and working south. These will make your orange into latitude slices.

Our world has 90° degrees of longitude either side of the equator. In Greenwich, London, there's a special latitude line called the "Meridian" - or latitude 0°. From the meridian the lines of latitude are numbered from 0° to 180° east and west. Now eat your orange!

Astro Navigation

With the detailed maps of the heavens and the aid of an instrument, probably not unlike a cross staff, Zheng He was able determine his latitude by measuring the altitude of Polaris (the Pole Star) to the north, and the teng lung ku star (Southern Cross) to the south. The navigators would calculate their position along a line of latitude and then sail along this line until they reached their destination.

For our astro navigation we have Steve's sextant and the nautical almanacs that contain all the necessary figures to make our calculations.

Zheng He used stars to navigate by. We use the sextant to measure the angle of the sun above the horizon, which is easier than using the stars. It's also a more accurate way of calculating latitude and longitude.

To see how little astro navigation has changed in 600 years have a look at the two diagrams here, the one on the left is from 1430 and the other is the one we use today. Both show Ursa Major, the plough, looking like a saucepan. If it's a clear night  - look out and see if you can spot it. If you follow an imaginary line - like the blue on the right side of the right diagram you should find Polaris, the Pole Star.

Modern Navigation aids

We also have stars that Zheng He never could have imagined ... satellites. They have very accurate clocks that send radio signals down to earth. The system we use onboard is called GPS (Global Positioning System). In addition to using the sun and stars - which we know where they are because astronomers have spent a huge amount of time to calculate their exact position relative to earth, we use small satellites which constantly calculate their own position and send it down to earth as radio signals. Our receiver onboard measures the time delay, in fractions of seconds, and uses its powerful computer to calculate our position. It is a similar mathematical operation as traditional astro navigation, but using modern technology. To the right you can see the instruments we have by our chart table, the leftmost being the GPS, showing our Latitude to be 2°10'.91. Look in an atlas or on a globe and try to guess where we are (hint - check the route page for where we have been so far..)

In the picture above you can also see two more instruments, a Depth sounder which shows how much water we have have under the keel and a Speed log which shows the boats speed through the water. We use the speed to calculate the distance we have travelled. These instruments are not linked to satellites but to transducers that go through the hull. The Depth sounder sends out a high frequency sound signal, which then bounces (echoes) off the seabed. The instrument then measures the time from when it sent the pulse to getting it back and calculates the distance. The speed log works by a little impeller (shovel wheel) that rotates as the water flows past it.

Most importantly, you need to know your Speed , Distance and Time to work out where you are. The figure on the right shows how to work out the missing one, hide the unknown value and you get the formula for calculating it.

Looking at the instruments in the photo above - what do you think we were doing when the picture was taken?

The Leadline

A lead weight attached to a long line that is marked with knots at regular intervals. The "leadsman" swings the lead over the side of the ship letting the line run out until it hits the sea bottom. He then pulls the line in and checks the knot closest to the waterline. As all the knots are different, he knows how much line has been let out and will shout to the captain the current depth. Not only is the depth of importance, if we want to anchor we like to get a nice sandy or muddy seabed for the anchor to dig into. By sticking a bit of tallow (sheep fat) to the lead, samples of the bottom may be brought up for examination.

We have a home made leadline made from an old rigging screw and a length of line marked at regular intervals with coloured silk and knots. Steve is our leadsman.

The Log

On Zheng He's voyages the speed of the ship was estimated by a sailor who threw a floating object overboard at the bow of the ship and walked back to the stern, reciting a formula that indicated the length of time taken. Simple. For example, if the boat was sailing at 2 knots (Nautical Miles per Hour) it would take 15 seconds to walk 150 feet. We have already checked our ships log twice with the same method, but we used a watch instead of the formula to measure time.