Sunday, 10 February 2008

Introduction to Navigation

Longitude and latitude. The earth can be regarded as a spherical object, and since we're dealing with a 3-dimensional shape we need coordinates of a different form than the usual x- and y-axes. Though adding an extra z-axes would make sense for submarines, we will most likely be found on the surface of this sphere while using another system of coordinates, that covers our planet with imaginary lines called meridians and parallels. All these lines together provide the grid which enables us to describe any position in longitudes and latitudes.

The obvious place to divide the Northern and Southern Hemispheres was the equator. But the division of the Eastern and Western hemispheres was the source of much political turmoil. Greenwich (Great Britain) won, placing for example The Netherlands in the Eastern and Ireland in the Western Hemisphere.

It takes the earth 24 hours for a full rotation of 360°. Thus, every hour we rotate 15°longitude.

When it is 12:00 Greenwich Mean Time (GMT) - anywhere in the world - it is 12:00 Local Time in Greenwich and 24:00 Local Time at the other side of the planet: 180° E or 180° W: the date line. Crossing this special meridian changes not only the hour but also the date.

The North Pole has a latitude of 90° N and the South Pole 90° S. The meridians cover twice this angle up to 180° W or E. Meridians converge at the poles, whereas parallels run parallel to each other and never meet. All meridians and the equator - the biggest parallel - form great circles, and the remaining parallels form so-called small circles. A great circle divides the earth in two exact halves.

Most sailors will actually notate seconds in metric fractions of minutes: 42° 21',5 N , 71° 03',6 W or 42° 21'.5 N , 71° 03'.6 W

On small scaled charts we want to be accurate within one minute or one nautical mile. On larger scaled charts the accuracy is more likely to be within a tenth of a mile (a cable).

If the earth were a perfect sphere with a circumference of roughly 40000 kilometres all great circles - meridians plus the equator - would have the same length and could be used as a distance unit when divided into 360 degrees, or 360° x 60' = 21600' minutes. In 1929, the international community agreed on the definition of 1 international nautical mile as 1852 metres, which is roughly the average length of one minute of latitude i.e. one minute of arc along a line of longitude (a meridian). Or to put it shortly: 1 nm = 1'

We are now able to describe any position in latitudes and longitudes. Moreover, we can state the distance between two of those positions using nautical miles or minutes. All we need now is a proper way to define speed. For that, sailors use knots, the number of nautical miles an hour.

Reproduced courtesy Bowditch Navigation

Map reproduced courtesy

You can read much more about navigation for sailors on my website

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