Direction, Latitude and Longitude 1 — Page 14, Lesson 20

I want to walk you through the foundation of the entire Latitude and Longitude system, because this is the skeleton that all of navigation hangs on. We've got the graticule on the globe in front of us — that's the network of lines you see drawn on a globe, the meridians running pole to pole and the parallels running east-west. Using that graticule, the position of any point can be accurately and unambiguously defined. That's the key promise: no ambiguity, one point, one unique address. But here's the crucial twist I want you to lock in right now. Position is defined by angular measurement — degrees, minutes, and seconds of arc — not by distance, as on Cartesian co-ordinates. On a flat graph you'd say "go 3 units right, 2 units up." On the Earth, we say "this many degrees of angle." That's the whole foundation of the Latitude/Longitude system, and it's why we need to be precise about angular units. So let's nail down the angular measurements. The fundamental unit is the degree, written with the symbol °. A degree is defined as the angle subtended at the centre of a circle by an arc equal to the 360th part of the circumference — that's why there are 360° in a circle. Think of it this way: if you slice the circle's circumference into 360 equal arcs, each arc, seen from the centre, subtends one degree of angle. For more accuracy, a degree is sub-divided into 60 minutes of arc, written with the symbol '. And a minute can be further sub-divided into 60 seconds of arc, written with the symbol ". So one degree equals 60 minutes, one minute equals 60 seconds. Now, a subtle but important point: these units — degrees, minutes, seconds — are mainly used for angular measurement of position. But for angular measurement of direction, it's more common to use degrees and decimals of degrees. So when you're talking about a bearing or a heading, you'll often see something like 127.5°, whereas when you're defining a position, you'll see 40° 30' 15". Keep that distinction in your head — position gets the full degrees-minutes-seconds treatment, direction usually gets decimal degrees. Now let's move to latitude itself. The latitude of any point is the arc — the angular distance — measured along the meridian through the point, from the Equator to the point. Let me unpack that. A meridian is a line of longitude running from the North Pole to the South Pole. So for any point, you follow its meridian down to where it crosses the Equator, and the angular distance along that meridian from the Equator up to the point is its latitude. It's expressed in degrees, minutes, and seconds of arc, and it's annotated North or South, depending on whether the point lies North or South of the Equator. Let's use the figure. In Figure 1.8, you're looking at the Earth 'sideways on' from space, with the North Pole at the top of the diagram. There's a point A, and its meridian crosses the Equator at a point labelled P. The angular distance between P and A is 40°, so the latitude of point A is 40°N. And there's an alternative way to say the same thing: we can say that A lies on the Parallel of Latitude of 40°N. A parallel of latitude is that east-west line circling the globe at a constant latitude — so point A sits on the 40° North parallel. So to tie it all together: latitude is an angle measured along a meridian from the Equator, expressed in degrees, minutes, and seconds, labelled North or South, and it places you on a specific parallel. That's the first half of your unambiguous position. The graticule gives you the framework, the angular units give you the precision, and latitude gives you your north-south address.