Direction, Latitude and Longitude — Page 8, Lesson 14

We're starting a brand-new topic: Direction, Latitude and Longitude. Let's begin with the Earth itself, because everything in navigation hangs off the shape and motion of the planet. First, a quick note on the Earth's shape. The real Earth isn't a perfect sphere — it's slightly flattened at the poles and bulges at the equator. That's why ICAO, the International Civil Aviation Organization, has adopted WGS 84 as the world standard. WGS 84 is a precise mathematical model of the Earth's shape, and modern navigation systems automatically correct position information for those distortions of the Earth's shape inside the navigation computers. So when you fly, the box is doing that correction for you. But for any calculations you'll do for the EASA examination syllabus, we simplify. We treat the Earth as a true sphere with a circumference of 21,600 nautical miles, or 40,000 kilometres. Keep those two numbers in mind — 21,600 NM and 40,000 km — because they'll come back again and again. Now, the Poles. The Poles are defined as the extremities of the axis about which the Earth spins. So imagine a rod running through the centre of the Earth, top to bottom — that's the axis of rotation, and the two ends of that rod are the Poles. There's an important detail here: the axis of the Poles is inclined to the axis of the Earth's orbit around the Sun at an angle of 23½ degrees. That tilt is what gives us seasons, and we'll cover it fully in the chapter on Time. For now, in this chapter, we draw the polar axis upright, as if it were vertical, just to keep things simple. Now let's define direction on the Earth. To start defining directions, we need a datum — a reference point to measure from. The simplest datum is the direction in which the Earth is spinning. That direction is defined as East. That's why the Sun rises in the East — the Earth spins toward the East. West is then defined as the opposite of East. Here's the key step. If you face East, the pole on your left is called the North Pole, and the direction North is defined as the direction towards the North Pole. The pole diametrically opposite the North Pole is called the South Pole, and the direction South is defined as being opposite to North. So North and South are defined by the poles, and East and West are defined by the spin. These four directions — North, South, East and West — are known as the Cardinal Points. Then we have the midway directions between them. Between North and East is North-East, or NE. Between East and South is South-East, or SE. Between South and West is South-West, or SW. And between West and North is North-West, or NW. These four are known as the Quadrantal directions. One more thing you'll need for solving navigation problems: sometimes you have to consider the Earth viewed from above either the North Pole or the South Pole. When viewed from above the North Pole, the Earth appears to rotate in an anticlockwise direction — that's counter-clockwise. When viewed from above the South Pole, the Earth appears to rotate in a clockwise direction. So from the top, it spins anticlockwise; from the bottom, clockwise. That's a handy fact to keep straight, because it tells you which way things move when you're looking down at the pole. So to recap: we've got the Earth as a sphere, 21,600 NM or 40,000 km around, spinning on an axis tilted 23½ degrees. East is the direction of spin, West is opposite, North is toward the North Pole, South is opposite. Cardinal points are N, E, S, W; quadrantal are NE, SE, SW, NW. And from above the North Pole, rotation is anticlockwise; from above the South Pole, clockwise.