Direction, Latitude and Longitude — Page 14, Lesson 19

I want to walk you through the very foundations of navigation — the grid we hang on the Earth so we can say exactly where we are. This is where Direction, Latitude and Longitude all begin, and it starts with one line: the Equator. Think of the Earth spinning on its axis — that's the polar axis, the line running through the North and South poles. Now imagine a Great Circle — a circle on the Earth's surface whose plane passes through the centre of the Earth — and that circle's plane sits at exactly 90° to that polar axis. That circle is the Equator. It runs East-West, and it splits the Earth into two equal halves, the Northern and Southern hemispheres. For defining position, the Equator is the datum — the reference line — for Latitude. It's the equivalent of the X axis in the Cartesian coordinate system you know from graph paper. Now the Meridians. A meridian is a semi-Great Circle — half of a Great Circle — that joins the North and South poles. Every meridian indicates True North-South direction. Here's a key point: every Great Circle that passes through both poles forms a meridian and its Anti-meridian — the opposite half of that same circle, 180° away. And the meridians cross the Equator at 90°, a right angle. Then we have the Prime Meridian — the one that passes through Greenwich, so it's also called the Greenwich Meridian. This is the datum for defining Longitude, and it's the equivalent of the Y axis in the Cartesian system. So you can already see the picture forming: the Equator is your X axis, the Prime Meridian is your Y axis, and together they give you a grid. Now, a Small Circle. A circle on the Earth's surface whose centre and radius are not those of the Earth — that's the definition. Its centre isn't the Earth's centre, and its radius isn't the Earth's radius. The main small circles relevant to position are the Parallels of Latitude. Those parallels are small circles whose planes are parallel to the Equator. They lie East-West, and their job is to indicate position North or South of the Equator. Finally, the Graticule. That's the network formed on a map or on the surface of a globe by the Prime Meridian, the meridians, the Equator, and the parallels of latitude. It's the analogy on the Earth's surface of the X-Y grid on graph paper. Let me show you what that looks like on a globe. So the whole system hangs together: the Equator and Prime Meridian are your two datums, the meridians give you True North-South, the parallels give you position North or South of the Equator, and the graticule is the complete network that results. That's the skeleton of all navigation — every position you'll ever plot sits on this grid.