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

Let’s start with the graticule. That’s the network of lines you see drawn on a globe — the lines running north–south and east–west. Using the graticule, the position of any point on the Earth’s surface can be accurately and unambiguously defined. But here’s the key difference from the Cartesian co‑ordinates you might have used in maths: position on the Earth is defined by angular measurement — degrees, minutes, and seconds of arc — not by distance in miles or kilometres. That angular measurement is the foundation of the entire latitude/longitude system. Now, let’s get the units straight. The fundamental unit of angular measurement is the degree, symbolised by the small superscript circle (°). 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 we have 360° in a full circle. For finer accuracy, a degree is sub‑divided into 60 minutes of arc, written with a single prime mark (′). And a minute can be further sub‑divided into 60 seconds of arc, written with a double prime mark (″). So one degree equals 60 minutes, and one minute equals 60 seconds. But here’s a practical distinction: those units — degrees, minutes, and seconds of arc — are mainly used for angular measurement of position, meaning latitude and longitude. For angular measurement of direction, such as a bearing or heading, it’s more common to use degrees and decimals of degrees, like 045.5° rather than 045°30′. Let’s move to latitude. The latitude of any point is the arc — the angular distance — measured along the meridian through that point, from the Equator up to the point itself. 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. In Figure 1.8, you’re looking at the Earth sideways on from space, with the North Pole at the top of the diagram. Let’s say we have a point labelled A. The meridian that passes through A crosses the Equator at a point labelled P. The angular distance along that meridian between P and A is 40°. So the latitude of point A is 40° North — written as 40°N. You can also say that point A lies on the Parallel of Latitude of 40°N. That parallel is the small circle running east–west at that latitude, but the latitude itself is the angular arc measured north from the Equator along the meridian.