Direction, Latitude and Longitude 1 — Page 14, Lesson 24

I want to walk you through a concept that often trips people up at first — the reversal of the apparent sense of longitude at the Greenwich anti-meridian. Let's look at what that means. We start on the Greenwich Meridian, which is 0° longitude. Standing there, the Eastern Longitudes are to your east, and the Western Longitudes are to your west. That feels natural. Now, travel around the globe to the 180°E longitude — that's the Greenwich anti-meridian, the line exactly opposite Greenwich on the other side of the Earth. Here's the key point: the direction of East has not changed. East is still 090° True — that's the direction of the Earth's spin. And West is still 270° True, the opposite direction to the Earth's spin. So the cardinal directions haven't moved. But here's where it gets confusing. When you're standing on the 180°E meridian, the Eastern Longitudes are now out to your left — that is, to your west. And the Western Longitudes are out to your right — to your east. So you end up with a situation where the Eastern hemisphere is to your west, and the Western hemisphere is to your east. That reversal can cause real confusion when you're solving navigation problems, so it's something to watch out for. Now, let's move on to a fundamental difference in principle between latitude and longitude. These two angular coordinate systems work quite differently. Lines of latitude are all parallel to each other — in fact, we normally call them 'parallels of latitude'. They never converge; they stay the same distance apart all the way around the Earth. Lines of longitude, on the other hand, emanate from a point — the North Pole — reach their maximum separation at the Equator, and then converge back to a point again at the South Pole. So longitude lines start at a point, spread out, and come back together. I find the book's illustration helpful here: think of latitude like slicing a pineapple — you get parallel rings. Longitude is like segmenting an orange — you get wedges that meet at the top and bottom. Now, when we give a position in latitude and longitude, there's a strict convention: latitude is always quoted first, and longitude second. So for New York, the position is 41°N 074°W. That's latitude first, then longitude. There are alternative forms that allow more precision by including minutes of arc. For example, you might see 41°00'N 074°00'W, or the shortened form 4100N 07400W. The minutes give you a finer division than just degrees. Let me give you a real example. The Aerodrome Reference Point — that's the ARP — for Oxford Airport is approximately 51°50'N, 001°19'W, or in the shorter form 5150N 00119W. That ARP is the mid-point of runway 01/19. If you need even greater precision, you can quote position in seconds of arc. For example, the precise position for the Oxford ARP is 51°50'12"N, 001°19'11"W, or 515012N 0011911W. That level of detail isn't normally required in these notes, but it's good to know it's available when you need it.