Direction, Latitude and Longitude 1 — Page 8, Lesson 16

Let’s start with the big picture. We’ve got the cardinal points — North, South, East, West — and those were fine for maritime navigation, where you could get away with saying “head northeast” or “southwest.” But air navigation needs precision. You can’t fly a jet on “sort of east.” So the book introduces the Sexagesimal system, which is just a fancy name for measuring direction in degrees — 360 degrees in a full circle. Here’s the key idea: we measure direction clockwise from North. North is 000°, East is 090°, South is 180°, West is 270°, and if you keep going all the way around, you come back to North at 360°. In practice, North can be written as either 000° or 360° — it’s a personal choice. But here’s a real-world catch: a runway pointing north would be labelled “36,” not “00.” That’s because runway numbers come from the direction you’re heading, rounded to the nearest 10 degrees, and 360 divided by 10 is 36. Now, when we say direction is measured clockwise from North, we have to specify which North. If the North datum is the direction of the geographic North pole — the actual top of the Earth’s axis — we call it True direction, and we use the abbreviation (T). So North is 000°(T), East is 090°(T), South is 180°(T), West is 270°(T). And here’s a rule you must never break: always use a 3-figure group. It’s 000°(T), not 0°(T), and 090°(T), not 90°(T). Why? Because when you’re transmitting over the radio — R/T, radio telephony — a single missing digit can be catastrophic. If someone says “27°,” is that 027°, 270°, 127°, 227°, 327°, 271°, 272°? You can’t tell. So any direction with less than three figures should be treated as suspect. The one exception is runway directions, which are given at 10° intervals to the nearest 10°. So a runway orientated at 273° is called RW 27, and one at 078° becomes RW 08. But beware — runway directions are normally given with reference to Magnetic North, not True North. That’s a distinction we’ll come back to. Finally, we need reciprocals. The reciprocal of a direction is the direction 180° opposite. If you’re travelling 060°(T), your reciprocal is 240°(T) — that’s 060 plus 180. If a runway has a direction of 353°(T), the reciprocal runway is 173°(T) — 353 minus 180. So the rule is simply: add or subtract 180, whichever keeps you in the 000 to 360 range. That’s the foundation. Direction on the Earth is measured in degrees clockwise from True North, always written as a 3-figure group, and reciprocals are just 180° away.