Direction, Latitude and Longitude — Page 14, Lesson 19

I want to walk you through the foundational concepts of direction, latitude, and longitude. This is where we define how we describe position on the Earth's surface, so let's take it step by step. First, we have the Equator. The Equator is a specific Great Circle — and a Great Circle is any circle on the Earth's surface whose plane passes through the Earth's centre. The Equator is the Great Circle whose plane is at 90° to the Earth's axis of rotation, which we also call the polar axis. So imagine the Earth spinning on its axis; the Equator sits exactly perpendicular to that axis. It lies in an East-West direction, and it divides the Earth equally into two halves — the Northern and Southern hemispheres. For defining position on the Earth, the Equator is the datum for defining Latitude. Think of it as the equivalent of the X axis in the Cartesian coordinate system you might remember from mathematics. Next, let's talk about Meridians. Meridians are semi-Great Circles — meaning they are half of a Great Circle — that join the North and South poles. Every meridian indicates True North-South direction. So if you stand on any meridian, it points you directly toward the geographic North Pole and the South Pole. Every Great Circle that passes through the poles forms a meridian and its Anti-meridian — that's the opposite half of the same Great Circle on the other side of the Earth. The meridians cross the Equator at 90°, meaning they meet it at right angles. Now, the Prime Meridian — also called the Greenwich Meridian — is the meridian that passes through Greenwich, England. It is the datum for defining Longitude, and it is the equivalent of the Y axis in the Cartesian system. So just as the Equator is our starting line for latitude, the Prime Meridian is our starting line for longitude. Let's move to Small Circles. A Small Circle is any circle on the surface of the Earth whose centre and radius are not those of the Earth itself. In other words, its centre is not the Earth's centre, and its radius is not the Earth's radius. The main small circles that are relevant to position are the Parallels of Latitude. The Parallels of Latitude are small circles on the Earth's surface whose planes are parallel to the Equator. They lie in an East-West direction. Their function is to indicate position North or South of the Equator. So as you move away from the Equator toward either pole, each parallel of latitude tells you how far north or south you are. Finally, the Graticule. The graticule is the network formed on a map or on the surface of a globe by the Prime Meridian, all the meridians, the Equator, and all the parallels of latitude. It is an analogy on the Earth's surface of the X-Y grid on graph paper. So just like graph paper gives you a coordinate system to plot points, the graticule gives us a coordinate system to plot positions on Earth. That figure shows the graticule on a globe — you can see the Equator, the meridians running pole to pole, and the parallels of latitude running parallel to the Equator. It really helps to visualise how all these elements fit together.