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 of the date line. Crossing this 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, where as 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. On small scaled charts you 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 one minute of arc along a line of longitude (a meridian). You should now able to describe any position in latitudes and longitudes. Now you can state the distance between two of those positions using nautical miles or minutes. All you need now is a way to define speed. For that, you use the term knots, the number of nautical miles an hour.

**Parallels:**Circles parallel to the equator, ranging from 0° to 90° N or S. Only the equator is a great circle.

**Meridians:**Half-circles converging at the poles, ranging from 0° to 180° E or W. Each pair of opposing meridians forms a great circle.

**Prime meridian:**0° or the Greenwich meridian which, together with the date line meridian, divides the Western and Eastern hemispheres.

**Great circle:**The intersection of a sphere and a plane that passes through the sphere's center.

**Small circle:**The intersection of a sphere and a plane that doesn't pass though the sphere's center.

**Time zones:**By convention 24 zones, each 15° longitude wide. Noon at Greenwich gives you midnight at 180° E.

**GMT:**Greenwich Mean Time, UTC or Zulu, which is the local time at Greenwich. For example, local time in Seattle, Wa. = GMT + 8.

**Date line:**The 180° meridian which extends from or is opposite to the prime meridian. Here, not only the hour changes when crossing the meridian, but also the date.

**Latitude:**Position defined by the number of degrees north or south of the equator, varies from 0° to 90°.

**Longitude:**Position defined by the number of degrees east or west of the prime meridian, varies from 0° to 180°.

**Position:**Latitude first and longitude second. For example: 42° 21'.5 N , 71° 03'.6 E.

**Nautical mile:**One nm is one minute (') on the vertical scale on the chart. 1' equals 1852 metres. Nautical miles are divided into 10 cables. A cable is equal to one tenth of a nautical mile.

**Knots:**Nautical miles per hour.